gamma spectroscopy principles

7/30/2019 Gamma spectroscopy Principles

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9. NUCE604 Revision:

Gamma SpectrometryPrinciples


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Gamma Spectroscopy Principles Page 1 of 35 NUCE 604

Gamma Spectrometry

PrinciplesAim

To introduce course members to the practical application of high-resolution gamma rayspectrometry and to explain the processes of detector calibration and of spectralinterpretation.

Objectives

At the end of this session the course members should be able to:

1 Draw a diagram showing the main components of a high-resolution gammaray spectrometry system.

2 Explain the concepts of live time, dead time and real-time in a countingsystem.

3 Explain the function of pole zero compensation.

4 Explain what is meant by pulse pile-up.

5 List five types of interference in gamma ray spectrometry.

6 Explain the effects of increasing count rates on a spectrum.

7 List five factors to be considered in selecting a counting geometry.

8 Explain the steps involved in calculating a sample activity from a gamma ray

spectrum.

9 List the criteria for accepting or rejecting a provisional nuclide identification.


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Gamma Spectroscopy Principles Page 2 of 35 NUCE604

Gamma SpectrometryPrinciples

1 Introduction

Whatever type of detector is used for gamma spectrometry the detector outputpulse height contains information on the gamma ray spectrum incident on thedetector. To maintain the best possible spectral resolution this informationmust be collected, displayed and analysed with the minimum of noise added tothe signal by the processing electronics. This lecture describes typical signalprocessing units used in high-resolution gamma ray spectrometry in todaysradiochemistry laboratory.

The first wide energy range high-resolution gamma ray detectors were

fabricated from lithium drifted germanium and were known as Ge(Li) detectors.These detectors have now been largely superseded by high purity germaniumdetectors (HPGe) also known as intrinsic or hyperpure germanium detectors.

Regardless of whether a Ge(Li) or HPGe detector is used very similarelectronics is used to process the data.

2 Detector Selection

When selecting a detector for a gamma ray spectrometer a philosophy of thebigger the better is not necessarily appropriate. If the detector is required for

general-purpose spectrometry then a detector with a good efficiency above1 MeV or higher is always required. The critical decision is the lowest energygamma ray that will be required to be detected. If the lower limit is about60 keV (suitable for Am-241) then the conventional HPGe detector (a p-typecoaxial germanium crystal) is probably suitable.

If it is required to extend the available energy range too much below 50 keVthen a thin detector end cap and a thin detector dead layer (or entrancewindow) is necessary. This will normally require a reverse electrode n-typecoaxial germanium crystal, although some p-type coaxial detectors withspecial thin entrance windows are available.

Both these types of detectors have very thin electrical contacts on the front

surface facing the incoming radiation, and so have a very low attenuation oflow energy gamma rays before the gamma rays reach the active volume.

For specifically low energy gamma ray or x-ray spectrometry a planar detectorwould be more suitable to cover the range 5 keV to 500 keV, however thisregion is not being separately covered.

A comparison of the efficiency curves for the various types of detector isshown in Figure 1.


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Figure 1 Typical Efficiency Curves for Ge Detectors

The other main consideration is the energy resolution required. For generalpurpose spectrometry the highest available resolution is not usually required;however, it must be remembered that good resolution implies a goodpeak-to-Compton ratio, and thus small peaks are easier to locate andmeasure. A resolution of 1.9 to 2.1 keV at 1332 keV is normally adequate.

Another useful parameter relating to the peak shape is the ratio of the peakwidth at one tenth of the maximum peak height (FWTM) to the peak width atone half of the maximum peak height (FWHM) and similarly the ratio at onefiftieth of the maximum peak height (FWFM) to FWHM. For a purely Gaussianpeak shape these ratios would be 1.82 and 2.38 respectively. For manycomputer based spectrometers, the program fits a Gaussian function to themeasured peak to determine the peak position and area, thus a true Gaussianpeak shape would produce a good mathematical fit. For typical moderndetectors of 30% relative efficiency, values for these parameters should beabout 1.95 and 2.8, although for bigger detectors (~100% relative efficiency)the values may be 2.0 and 3.0 and for smaller detectors (10% relativeefficiency) about 1.90 and 2.65.

For radiochemical measurements the most convenient geometry for thedetector and cryostat would probably be a vertical dipstick geometry with thedetector placed vertically above the liquid nitrogen Dewar. This allowssamples to be placed vertically above the detector at suitably predefinedpositions on a jig either inside a lead shielded enclosure or with no externalshielding. This would allow a wide range of activities to be measured with thesame detector.


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Any shield used should allow at least a 10 cm air gap between the shield walland the detector and source. The usual shield material is 10 or 20 cm of lead,and the characteristic lead X-rays at 75 and 85 keV will be observed in anyspectrum. To overcome this the shield must be lined with a lower Z material toprevent the excitation of the X-rays and to absorb any that are generated. Theusual liner is a combination of 0.5 mm and 1.5 mm copper; this combinationwill produce Cu X-rays at 8 keV, which should not be a problem for mostgamma spectrometry applications.

Detectors have traditionally been cooled by being mounted on a cryostat that iscooled by liquid nitrogen. It is now possible to cool the detector without liquidnitrogen by using a closed cycle helium refrigerator. These are claimed to bereliable in operation and only require a maintenance period of 1 hour every10,000 hours (less than 1 hour per year). The experience of these deviceswithin Nuclear Electric has not been encouraging so far.

3 Electronics

The basic electronic components of all conventional high-resolution gammaspectrometers are very similar. This section of the report gives details of theelectronic building blocks shown in Figure 2 of a typical system. The mainsignal path is emphasised in the figure. The signal from the detector isamplified and shaped in the preamplifier and main amplifier to give an outputpulse whose height is proportional to the energy deposited by each gamma rayin the detector. These pulses are then converted to digital form in ananalogue-to-digital converter (ADC) and stored in a multi-channel analyser(MCA). The MCA sums the number of pulses of each height coming from theamplifier to produce a histogram of the number of events against pulse height(or energy); this is the gamma spectrum.

Figure 2 Typical Electronic Configuration for High Resolution Gamma

Spectrometer


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4 Preamplifiers

Modern germanium detector systems use a charge sensitive preamplifier,which normally comes as an integral part of the detector assembly. The lownoise input stage is usually a field effect transistor (FET) that is sometimesmounted with the detector inside the detector cap to keep it cool. This reduceshe electrical noise but is more difficult to repair or replace if it fails.

Figure 3 RC Feedback Preamplif ier

This first stage of the preamplifier has an RC feedback (resistor, capacitor)

network that provides an output pulse with an amplitude proportional to theintegrated charge output from the detector (see Figure 3). The pulse risetime(20 200 ns) is a measure of the charge collection time in the detector and thefall time (~1 ms) is controlled by the RC time constant.

The second stage is a differentiating circuit which produces output pulses witha rise time of about 10 ns and a fall time of ~50 s. A simple differentiatingcircuit would produce an output pulse with a long undershoot below thebaseline (see Figure 4). To maintain a steady baseline a pole zerocompensation network is used with the differentiating circuit to eliminate thisundershoot. It consists of a variable resistor across the differentiatingcapacitor. This circuit is usually factory adjusted and requires no furtherattention.


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Figure 4 Preamplif ier Behaviour RC Coupled

The sawtooth waveform of the input signal means that even at low count rates(~ 1000 cps) most pulses will be sitting on the tail of the previous pulse,causing the mean DC Level to rise slightly. As long as the peak of the secondpulse does not exceed the dynamic range of the preamplifier the output pulse

is still a true representation of the input gamma energy. At higher count ratessome pulses may saturate the preamplifier, see Figure 5, and the output pulsewill be distorted. At even higher count rates the mean DC level may exceedthe dynamic range and then there will be no output until the count rate drops.This lack of output is the shutdown effect.


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Figure 5 High Count Rate Effects in RC Coupled Preamplif ier

The shutdown effect is controlled by the energy rate limitation of thepreamplifier, or the product of the gamma ray energy and the input count rate.A typical value for the energy rate capability is 2 x 10

5MeV/s. Higher count

rates can be accommodated by having a shorter time constant in the RCfeedback circuit and thus having a shorter decay time. However, the penalty isan increase in the noise, which results in an increase in the spectral peakwidth. The energy rate product could be increased tenfold for an increase of ~20% in the peak width. This may be acceptable if very high count rates areexpected.


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An alternative solution to preamplifier shutdown is to use a transistor resetpreamplifier (TRP). In this type of preamplifier the feedback resistor in the firststage is replaced by a transistor (shown as a switch in Figure 3) thatdischarges (or resets) the feedback capacitor when the DC level starts toapproach saturation. As there is no decay of charge via the resistor, the signalon the capacitor builds up in a series of steps and there is no need for asecond stage differentiator with pole zero compensation.

This type of preamplifier never saturates and so does not shutdown, howeverthere is a small time period associated with each reset pulse during which it isnot able to accept any input pulses. This dead time must be taken intoaccount when calculating the input pulse count reset rate. This reset alsoaffects the main amplifier, the negative reset signal drives the amplifier intooverload and the amplifier may be out of action for about 3 pulse widths,i.e. for the time it takes to process 3 pulses.

Figure 6 Signal Behaviour with TRP


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A TR preamplifier gives a better resolution as there is no feedback resistor,and for the same reason it has a higher count rate performance. There is oneother important consideration; as the output is a step function rather than a tailpulse, there is no need for pole zero cancellation in the main amplifier,simplifying the amplifier set up procedure.

5 Main Ampli fierThe main amplifier has to be a signal processor as well as a signal amplifier.The output from a preamplifier either has a long tail pulse (from a RCpreamplifier) or a staircase ramp (from a TRP); both of these outputs havefloating baselines. The first stage of the amplifier has to clip the signal toproduce a pulse with a fast fall time and block the DC signal to bring thebaseline rapidly back to the reference potential. As with the RC preamplifier adifferentiating circuit with pole zero cancellation is used to return the pulsequickly to the baseline with little or no overshoot.

The first stage is usually followed by an integrator to recover the energyinformation from the signal that is contained in the pulse height.

In these first two stages compromises must be made between short timeconstants (to return the signal rapidly to the baseline), which allow too muchnoise, and long time constants (to improve resolution by reducing noise), whichincreases the possibility of pulse pile-up and consequent baseline shifts. Atypical optimum time constant for low count rates is 2-6 s, tending to thelonger time constants for larger detectors and TR preamplifiers.

To ensure good energy resolution and peak position stability at higher countrates (>10,000 cps) the various stages of the amplifier are DC coupled. Thusany DC offsets in the early stages can cause large offsets at the output.A baseline restorer is required to remove these offsets and ensure a steadyreference base line at ground potential.

This is usually carried out with a gated baseline restorer, which is a CR(capacitor, resistor) differentiating circuit with an open/close switch in theresistive leg to ground. This switch is open whenever a pulse is detected andclosed otherwise. Thus the differentiator is only active between pulses andkeeps the baseline close to ground potential. This method can keep thebaseline steady at input count rates up to 10

5cps with 2 s shaping time

constants.

At high input count rates two gamma rays may arrive at the detector within theresolution time of the amplifier. The two pulses will be piled-up and they willcreate a summed peak that looks like a single gamma pulse. For detectorswith a short charge collection time, i.e. having a short rise time, it is possible toreject these pile-up peaks using a pile-up rejecter (see Figure 7).


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Figure 7 Operation of Pile-Up Rejecter Circuit


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Each time an incoming pulse is detected a fast logic pulse is produced whichtriggers an inspection interval that covers the amplified pulse. If a second fastlogic pulse from a second event occurs during the inspection interval a signalrejection pulse is generated. This signal rejection is used in the ADC to rejectanalysis of this signal and the dead time of this system is extended to coverthis second signal.

In order to count at very high count rates (input count rate >50,000 cps) it isusual to reduce the shaping time constant no less than 2 s. However, withlarge detectors the charge collection time can be long (up to ~500 ns) andvariable. This results in a long and variable rise time in the preamplifier pulsesand causes lower amplitude main amplifier pulses because not all charge willbe collected in the pulse processing time. This effect is known as ballisticdeficit. The ballistic deficit causes an increase in the width of spectral peaksand may be significant as the shaping time constants are reduced below 2 s.

A gated-integrator amplifier can overcome this ballistic deficit problem.Instead of a RC shaping integrator in the second stage of the amplifier there isa circuit that integrates the total charge from the detector for each pulse fromthe first stage. When the input pulse signal returns to the base line theintegrating circuit is discharged, ready for the next pulse. This process gives aslow rising pulse with a sharp cut-off whose peak is much later in the pulsethan in a Gaussian pulse with the same time constant. This enables pulses ofvariable risetimes to be accurately processed and avoids the problems ofballistic deficit.

Figure 8 shows how the gated integrator output pulse produces good resultsfor slow rise time input pulses that would give problems of ballistic deficit with aconventional gaussian amplifier.

The gated integrator amplifier can give good results at time constants as shortas 0.25 s with input could rates up to 5 x 10

5cps.

The amplifier must be able to accept the inhibit signal from a TRP during thereset interval to prevent the processing of any pulses in this period and add tothe total system busy time.

As well as the amplified output signal, there are two output logic signals fromthe amplifier one to indicate the presence of pulse pile-up and another toindicate the time the amplifier was busy and not able to process other pulses;these signals were indicated on Figure 7.


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Figure 8 Improved Performance Using a Gated Integrator


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6 ADC

An analogue to digital converter (ADC) measures the maximum amplitude if ananalogue pulse and converts it to a digital (or channel) value. The digitaloutput is proportional to the pulse amplitude and thus proportional to theenergy of the initiating gamma ray. For a series of pulses the digital outputsare stored in a multi-channel array that gives the cumulative number of countsobserved in each channel. This is done by adding one to the contents of thechannel corresponding to the pulse height measured. The resulting histogramrepresents the spectrum of input pulse heights. If the input pulses come froman energy spectroscopy amplifier the histogram corresponds to the energyspectrum observed by the detector connected to the amplifier. The histogramstorage device can be a dedicated memory, or part of a computer memory.With the addition of a histogram display device the combination of ADC,memory and display is known as a multi-channel analyser.

These are two major types of ADC used in high-resolution gamma rayspectroscopy, the Wilkinson type ADC and the successive approximationADC.

7 Wilkinson ADC

The operation of a Wilkinson ADC is shown in Figure 9.

A pulse is detected when the signal level exceeds the pre-set lower leveldiscriminator (LLD). The LLD is usually set just above the noise level of theamplifier to prevent the ADC permanently analysing noise. The LLD triggersthe linear gate and peak detection circuit, during this time period a specialcapacitor, the rundown capacitor, is connected to the input signal. Thiscapacitor is forced to charge up so that its voltage follows the amplitude of theinput pulse. When the peak of the signal is detected the linear gate is closedand the rundown capacitor is disconnected from the signal input. At this time,the capacitor has the same voltage as the maximum voltage of the signalpulse.

When the linear gate is closed a constant current source is connected to therundown capacitor to discharge it, at he same time a clock (the conversionclock) is started and the clock pulses are counted until the capacitor voltagereaches zero. The number of clock pulses is proportional to the pulseamplitude and represents the channel number where the event is to be storedduring the memory write cycle.

During the time the pulse is being processed, the ADC cannot process anotherpulse and so it produces a so-called busy signal equal to the processing timeperiod. The busy signal indicates that the ADC is dead to any further pulseswhile it is analysing any single pulse. In order to measure the true detectorevent rate these dead time losses must be corrected.


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The correction is usually done by counting clock pulses from an oscillator.Only when the ADC is not busy are these clock pulses stored, this time isknown as the live time. If this live time is used as the count duration period(rather than the actual count duration or real time) the dead time losses areaccounted for.

The ADC dead time depends on the conversion clock frequency, (the higherthen frequency the faster the rundown capacitor can be discharged), theamplifier pulse height (or storage channel number) and the memory write cycletime.

TMF

NDT +=

DT is the dead time.N is the channel number.F is the conversion clock frequency.TM is the memory write cycle time.

For modern ADCs conversion clock frequencies of 50 to 450 MHz are typical

and memory write cycle times carry from 0.5 to 2 s.

Table 1 gives the conversion times for several clock rates for a differingnumber of ADC storage channels.

As can be seen from the table, for a typical high resolution gamma rayspectrometer with a 100 MHz ADC and 4096 storage channels the conversiontime of 43 s for a pulse in the highest channel is longer than other timesinvolved in pulse processing. Typical pulse processing times for an amplifierare about 8 times the shaping time constant or 16 to 32 s for a conventionalgaussian amplifier or down to 4 s for a gated integrator amplifier used at highcount rates.

However the amplifier busy times are not significant and these dead timeshave to be summed with the ADC dead time to obtain the overall system deadtime. Most ADCs have a facility for accepting the amplifier dead time andlogically Or-ing this with the ADC dead time.


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Figure 9 Operation of a Wilkinson Type ADC


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8 Successive Approximation or Fixed

Conversion Time ADC

The major part of the process in this type of ADC is similar to a WilkinsonADC. A capacitor is charged up by the incoming signal, but instead of beingdischarged when it reaches its maximum voltage, it is held at this voltage. Avoltage comparator then measures the voltage on the capacitor.

The voltage comparator is a successive approximation device. The mostsignificant bit of a digital to analogue converter (DAC) is set, the comparatordetermines if the DAC voltage is greater than the capacitor voltage, if so themost significant bit is reset to zero, if not the bit is left set. Subsequently thesame test is made by setting the next significant bit. This process is repeateduntil all the bits have been tested. The bit pattern at the end of the process isthe channel number to store the input pulse.

If the ADC has n bits (i.e. 2n

channels) n test cycles are required, which is the

same for all pulse amplitudes. This type of ADC has a short conversion timethat is independent of pulse amplitude, but the linearity may not be as food asa Wilkinson type. Conversion times of 1.5 s to 10 s are available. Table 2gives a range of conversion to compare with Table 1.

Clock

Rate MHz

Conversion Time sincluding memory

Cycle

Number of Channels, N

1024 4096 8192

50 0.02 x N +2 22.48 83.92 165.84

100 0.01 x N +2 12.24 42.96 83.92

450 0.0022 x N +2 4.28 11.10 20.20

Table 1 Conversion Times for Wilkinson ADCs

Nominal

Conversion

time s

Actual Conversion

Time with Memory

Cycle s

Number of Channels, N

1024 4096 8192

8 9.5 +2 11.5 11.5

8.55 6.5 +2 8.5 8.5

1.5 2.4 +2 4.4 4.4

Table 2 Conversion Times for Successive Approx imation ADCs

For a typical 4096-channel gamma spectrometer the 5 s fixed conversion

time (FCT) ADC would be faster than a 100 MHz Wilkinson type, but wouldonly be faster than a 450 ADC at the top of the range. To compare theconversion times of the various ADCs see Figure 10.


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For most spectra the majority of the events are of low energy due to the largenumber of Compton events, which contribute to the low energy continuum, andthe efficiency of HPGe detectors being much higher at low energies (~100keV)than at high energies (~1 MeV). To obtain the highest pulse throughout a 1.5s FCT ADC should be used with a 4096-channel spectrometer.

Figure 10 Comparison of Conversion Times for Wilkinson and Fixed

Conversion Time ADC

9 High Voltage Power Supply

All germanium gamma ray selectors require a high voltage bias supply. Avariable 0-5 kV supply with a low current capacity (


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10 Liquid Nitrogen Level Monitor

This is not essential, but it is a useful accessory. The monitor will inform theuser, via an audible alarm that the liquid nitrogen level in the Dewar is low andneeds refilling, before the detector warms up. The unit will also turn off theEHT.

The nitrogen level is monitored by a thermistor probe in the Dewar, which isconnected directly to the monitor. When the Dewar is approximately full thenitrogen level is below the thermistor and it warms up creating the audiblealarm.

11 Digital Spectrum Stabiliser

It is often suggested that it is necessary to stabilise spectra to prevent zerointercept and gain variation in the spectrometer, which may degrade aspectrum, collected over a long time. In most simple gamma spectrometrymeasurements, where the half life of the isotopes are long compared with thecounting time, the system count rate will not vary significantly during the countperiod, and the gas in unlikely to change. This may not be the case in neutronactivation analysis measurements where there may be gain shifts due to countrate variation.

There may be long term zero and gain drift bit this can be corrected by thesystem operator manually recalibrating the system as required. For atemperature controlled counting laboratory the energy calibration should besufficiently stable for periods of days or weeks.

Spectrum stabilisers rely on the presence of two peaks in the spectrum, one ata low energy and the other at a high energy, these peaks are then maintainedin fixed channel positions by small variations in the system zero offset andgain. These two peaks can be provided in two ways.

1. A source or sources are placed close to the detector to provide suitablepeaks (they may in fact be part of the measured spectra itself, an internalenergy calibration).

2. Inject pulses of the required height into the preamplifier.

Both these solutions have problems, the first method means that the addedsources will always be present, increasing the background count rate and sodecreasing the minimum detectable activity of all isotopes. The isotopes thathave been added will also be difficult to determine if they occur in the sampleto be measured.

The second solution is only as good as the stability of the pulsar, anddifficulties with pole zero cancellation of the pulsar tails may lead to somebroadening of all peaks.

In the special case of continuous monitoring of an industrial process whereknown isotopes are being measured, (such as the on-line coolant monitor onAGR) there may be some advantage in using a digital spectrum stabiliser.


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12 Setting Up a Gamma Spectrometer

In summary, the setting up procedure consists of adjusting the main amplifiergain to give a maximum pulse height suitable for the full range of the ADC.The pole-zero of the amplifier is adjusted to minimise the spectral peakswidths, and the baseline restorer optimised for the maximum count rateexpected. The ADC is switched to the desired conversion range and adjustedto minimise any DC offset from the amplifier. The controls may then all beslightly readjusted in turn to improve the system performance. It is importantthat the adjustments are made in a logical sequence; otherwise the fullcapability of the system may not be achieved. A detail description is given inthe rest of this section.

This section will describe how to set up a typical high-resolution gammaspectrometer to cover an energy range 0-2.4 MeV with a 4096 channel ADCand multi-channel analyser. This is a common arrangement, and minorchanges to the procedure can be made to cover energy ranges.

13 Connecting the Electronic Units

The electronic units should be placed into a single electronics rack andpowered from a single clean mains power supply. The units should beinterconnected as shown in Figure 2 or as indicated in the manufacturesinstruction manual.

All signal cables should be of the same impedance to minimise signalreflection, preferably they should be:

93 cable (RG-62A/U) as this is a common output impedance frompreamplifiers.

The system should be dc-coupled from the preamplifier to the MCA. If anymodules have options, select dc-coupling rather than ac-coupling.

Apply power to the electronics to allow them to temperature stabilise, but donot apply bias to the detector.

The detector Dewar should be filled with liquid nitrogen and left for at least sixhours to allow the detector to cool.

14 Applying Detector BiasThe first time a new detector is used care should be taken to check it isperforming correctly. Connect the unipolar output from the main amplifier toan oscilloscope and adjust the gain to display the system noise. Any 50 Hzsine wave indicates a ground loop or other mains pick-up, the source shouldbe tracked down and eliminated.


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Apply about 100 V of bias of the correct polarity as indicated by the detectormanufacturer, (usually indicated on the detector end cap). The noiseamplitude on the oscilloscope should decrease; sometimes the noisedisappears completely to several seconds before reappearing. Increase thebias in steps of 100 to 200 V while observing the system noise. Allowing forthe amplifier gain the system noise should reduce to less than 1 mV. Whenthe bias voltage has been raised to the value given in the manufacturers datasheet the noise may have marginally increased.

Inspect the oscilloscope trace for any further problems due to ground loops,pickup or microphonics and eliminate these as much as possible beforeprocessing any further.

On subsequent occasions the full bias voltage may be switched on without riskof damage as most detectors have a HV filter with a long time constant.

15 Preampli fier Set-Up

With RC feedback preamplifiers no adjustment is required; the pole-zerocompensation is factory set and should not normally be adjusted. Somepreamplifiers have two gain settings, the higher gain is usually only used forlow energy gamma ray or X-ray application and the low gain setting should beselected for the current set-up.

The transistor reset preamplifier has an associated circuit for producing theinhibit pulse to fate (or reject) any signal pulses during the reset period. Thelength of this inhibit pulse must be adjusted to suit the gain setting on the mainamplifier as directed in the manufacturers handbook. The inhibit pulse is ofthe order of 50 to 200 s wide.

16 Main Amplif ier Set-UpTo provide suitable pulses to allow the amplifier to be set up adjust the coarsegain to 50 and the fine gain to 1.0, final adjustments can be made when all theunits are set up. Set the shaping time constants for both the unipolar andgated integrator output (if applicable) to 2 s. Set the input polarity switch tothe correct polarity, normally positive for p-type detectors with positive bias. Ifa TRP is used the amplifier pole-zero compensation potentiometer should beadjusted completely out. For a RC feedback preamplifier the pole-zeropotentiometer must be correctly adjusted. The operators manual for theamplifier should give the front panel switch positions to give the best pole-zeroset up conditions.

Place a 60Co source close to the detector to give a count rate of about 1000cps, connect the unipolar output of the amplifier to the input of theoscilloscope. Adjust the pole-zero potentiometer to bring the long tailcomponent of the trailing edge of the amplifier pulses to a flat baseline with nolong negative or positive tail (Figure 11). Remove the source from thedetector.


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If there is a dc-level adjustment keep the oscilloscope on the unipolar outputand adjust the dc-level to 0 1mV.

The base line restorer (BLR) must finally be adjusted. There may be anautomatic position and this can be selected, but for optimum performance athigher count rates select the variable BLR setting. There is normally an LEDor other indicator associated with the BLR adjustment; alter the BLRpotentiometer until the LED is on full, then back off the potentiometer until theLED noticeably dims and flickers. The BLR is now in about the best position.If a counter-timer is available the BLR can be adjusted while monitoring theBusy output with the counter timer. A count rate of about 200 cps gives thecorrect BLR setting.

Some amplifiers have a pile-up rejecter discriminator setting; this can beadjusted in a similar manner to the BLR using the LED method.

17 ADC Set-Up

In order to have channel zero corresponding to 0.0 MeV it is necessary toensure the dc-level of the ADC input is at zero volts. In some ADCs the inputdc-level can be adjusted, in others it is fixed at a nominal zero volts. Thissection will describe setting up an ADC with a variable dc-level.

Set the conversion gain to 4096, remove the signal input cable and insert ashorting plug into the ADC input. Set any coincidence/anti-coincidence switchto coincidence or, off, as all input pulses are to be accepted and sample theinput signal (0.0 v) either with the strobe switch or the sample voltage analysis(SVA) switch, dependent on the manufacturer. Set the lower leveldiscriminator (LLD) to its minimum value.

Expand the region of display to about the first 100 channels so that thecontents of each channel can be easily seen and initialise data acquisition.Adjust the ADC zero level control until data is accumulating in only the firstchannel. Remove the shorting plug and reconnect the signal input cable.Reinitialise data acquisition and check that data are still being acquired in thefirst channel. At this stage the signal may have noise associated with it andthe data may appear in several channels, but the peak should be in the firstchannel. If the noise peak is not in the lowest channel adjust the amplifierdc-offset or the zero control of the ADC if necessary.

Place an241

Am and60

Co source close to the detector to give a count rate ofabout 1000 cps and return the strobe or SVA switch to its normal position.Adjust the LLD control so that any noise peak in the lowest channel iseliminated but the peak due to the

241Am at 60 keV is detected. The LLD

control should be as low as possible, preferably giving a cut-off at aboutchannel 20 to 40. The upper level discriminator (ULD) can be set to itsmaximum value.

The amplifier gain can now be adjusted so that the60

Co 1332 keV peak is inchannel 2220 and the ADC zero level adjusted so that the

241Am peak at

59.5 keV is in Channel 100. These steps may have to be repeated severaltimes until both conditions are satisfied. If the amplifier gain is alteredsignificantly then the BLR may have to be reset.


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Figure 11 Pole Zero Compensation


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18 System Resolution Determination

Unless high count rates are to be routinely measured the shaping timeconstant of the amplifier should be adjusted to give the best resolution.Optimum performance at high count rates and best resolution at lower countrates will usually not occur at identical shaping time constant settings andsome trade-off in performance may be necessary to find a suitablecompromise for all count rates.

Place a57

Co source on the detector to give a count rate of about 1000 cps andacquire a spectrum with at least 3000 counts in the peak channel of the 122keV line (at least 10,000 total count in the peak). Analyse the spectrum andrecord the FWHM for the 122 keV peak. Repeat this process for a

60Co

source and the 1332 keV peak.

Determine the FWHM for both peaks for all shaping time constants (or asmany as seems appropriate) but check the main amplifier (in particular thepole-zero settings) and ADC settings are satisfactory between each change intime constants.

A plot of FWHM as a function of shaping time constant should give a graphwith a minimum value for the FWHM in agreement with the manufacturersdata or test sheet. The FWHM should increase rapidly at small time constantsand increase more slowly at longer time constants.

Normally the FWHM minimum value will be at a shaping time constant of 4 to6 s. The resolution should only deteriorate by about 10% at 2 s and lessthan 25% at 1 s.

For counting at rates of less than 1000 cps the optimum shaping time constantwill be satisfactory, but if routine is at rates in excess of this then shorter timeconstants should be used. For count rates up to 10,000 cps a shaping time of2 s will probably be adequate, and 1 s would probably be advisable for countrates up to 5 x 10

4cps. At this time constant a gated integrator amplifier would

probably give equivalent or better results. At input count rates above 5 x 104

cps a gated integrator with a 0.5 s or smaller time constant would probably bethe best option.

It is probably best to set the time constant to suit the most commonly usedcount rate and then calibrate other counting geometries at greater source todetector distances to reduce the count rate to the usual value rather thanchange amplifier settings from count to count.

19 Pile Up Correction

If more than one gamma ray is detected within the resolution time of theamplifier then the pulses are summed. This summed peak would appear inthe spectrum in a higher channel than the two individual gamma rays, this ispulse pile-up. The effect is to produce extra peaks in the spectrum andremove counts from the contributing peaks. These summed pulses should berejected by a suitably fast pile-up rejection circuit, but there will be some pulsesthat are sufficiently close in time that they cannot be rejected. Thespectrometer should be checked to determine the upper count rate at whichthe pile-up rejection circuit works adequately.


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Place an241

Am and a88Y source close to the detector to produce a count rate

of about 1000 cps. Acquire a spectrum with at least 104

counts in the59.5 keV, 898 keV and 1836 keV gamma ray peaks. Analyse the spectrumand determine the peak areas and the spectrum live time and real time.

Keeping the241

Am and88Y sources in the same fixed position bring up a

source of137

Cs and record spectra over a wide range of input count rates.Analyse the spectra and determine the peak areas and system times.

Figure 12 Pulse Pile-Up Correction Calibration


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Plot the ratio of the observed peak count rate to the actual peak count rate asa function of log (real time/live time) for all three peaks (Figure 12). The actualpeak count rate is defined as the count rate at the lowest count rate used i.e.without the

137Cs present.

The points should form a straight line for each gamma ray peak with a smallvalue for the slope. The slope should be similar for each peak and the meanslope can be used to correct spectra for high count rate losses from aknowledge of the system live and real time. In some computer basedspectrometers a function identical to or similar to this model is built in toautomatically correct all spectra.

At very high count rates the relationship may become non-linear and thespectrometer should not be used at count rates greater than this limit as theaccuracy will be reduced.

20 Count Rate Effects

The time that a detector is running is called the Real Time. The time that theADC was unable to accept signals as it was processing another pulse is calledthe Dead Time. The time the system was waiting to accept a signal is calledthe Live Time.

If the same isotope is counted on the same detector then the spectrumobserved will vary with the sample activity and the presence of other isotopes.At the lowest sample count rates, the rates normally encountered withenvironmental samples, the spectrum will be dominated by the detectorbackground count rate. The sample and detector should be in a lowbackground facility, preferably in a laboratory away from any source of highactivity.

The background spectrum inside a typical low background lead shield isnormally a combination of daughter products from 238U and 232Th decay.These daughters include

228Ac,

212Pb and

208Tl. There will also be

40K from any

concrete nearby and probably daughters of226

Ra that are transported viagaseous

222Rn released into the atmosphere. The latter component can be

reduced by venting the detector shield with the nitrogen gas boiling off from theDewar thereby keeping a flow of gas out of the shield.

Low count rate spectra will probably have been collected for a long time sothere will be a possibility of gain drift; the peaks could be both small and broad,making them difficult to locate and quantify. If only a few known isotopes areto be determined it would be best to look for these isotopes using a librarydirected peak search program that forces a fit to any peaked feature in thespectrum close to the location of the known isotope peak.

At normal count rates there will be little or no environmental backgroundinterference, the background for most peaks will be the Compton continuumfrom higher energy peaks. The peaks should be well defined and simple todetect except for minor constituents in a mixed isotope source. Any peaksearch program should be able to find all the peaks. To minimise analysistime the nuclide library used to identify the isotopes present should match thesample, i.e. if activation products only are being measured then the libraryneed not contain fission products.


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As count rates increase above about 1000 cps the system dead time increasesand good live time correction is required to give accurate quantitative results.

At count rates above 5000 cps or so the spectral peaks start to broaden andshift due to baseline variations, the random coincidence summing (pulsepile-up) will start to increase and extra pile-up peaks will occur in the spectrum.There will initially be pile-up between two identical gamma rays of the highestintensity, giving a pile-up peak at twice the energy, subsequently there will be apile-up between other intense peaks. If activities need to be determined tobetter than 5% corrections will have to be made for pile-up losses.

At higher count rates, say above 20,000 cps, peak broadening and peakshifting can be significant, depending very much on the set up of the individualspectrometer. The longer the shaping time constant, for either a unipolaramplifier or gated integrator, the earlier the effects will occur at increasingcount rates.

At the highest input count rates of 100,000 cps or more the output count ratewill drop, the system throughput being limited by the amplifier time constantand pulse pile-up in the amplifier. At some input count rate a RC feedbackpreamplifier will lock up completely due to its energy rate saturation and theoutput count rate will drop to zero. This effect does not happen with a TRpreamplifier, the output count rate just becomes smaller. At these highestcount rates the pile-up effects are very noticeable, the peaks may become verybroad with pile-up between peaks and Compton continuum pulses givingdecreased peak to Compton ratios.

21 Interferences in Gamma Ray

Spectrometry

The major advantage of the HPGe spectrometer over a NaI spectrometer is

the very much-improved resolution (~2 keV at 1332 keV for HPGe and about60 keV for NaI). At first sight this resolution improvement may suggest thatthere should not be any problems with differentiating one gamma ray energyfrom another in a well set up spectrometer. However, as many isotopes emitseveral gamma rays a multi-isotope source may have gamma rays from oneisotope that are not resolved from the gamma rays of other isotopes. Thereare several ways in which one isotope may suffer interference to one or moreof its gamma rays.

1. Background

In low count rate environmental samples the largest source of interference isusually the presence of the isotope to be measured in the detector and shield

background or in the blank sample. Typical problems here are the presenceof

60Co and

137Cs in the background due to contamination and nuclear

weapons fallout. Other problems arise if238

U and234

Th and their daughtersare to be measured. These isotopes are nearly always present as impurities inlead shields. In all cases care must be taken to subtract the correctbackground component. With large volume high-density samples the shieldingeffect of the sample may be significant and the background count must bedetermined with an equivalent blank sample.


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2. Compton continuum

At normal count rates the biggest source of interference is probably thepresence of other more active isotopes giving a large Compton continuum inthe region of the peaks of the isotope of interest. This problem can beimproved by counting for a longer time so that the Compton continuumbecomes better defined and smoother so that any small peak will be moresignificant. The only other solution is chemical separation of the isotopes.

3. Escape peaks

At any energy E greater than 1 MeV it is possible for an incident gamma ray tointeract with the detector to produce two annihilation gamma rays of 511 keVeach. If either or both of these gamma rays are not fully absorbed in thedetector then extra escape peaks are created in the spectrum. These peaksappear at energies of E-511 keV (the single escape peak) and E-1022 keV(the double escape peak).

The escape peaks may be confused with energies of interest. A typicalexample is the single escape peak of the

60Co 1173 keV peak at 662.2 being

confused with the 661.6 keV peak of137

Cs. All spectra with one or moreisotopes having a high activity with a gamma ray greater than 1 MeV should bechecked for escape peaks.

The annihilation gamma ray at 511 keV is of course not a reliable indicator ofany particular isotope as it is observed in spectra of many isotopes. A biggerproblem is that this peak interferes with other gamma rays such as 514 keVfrom

85Sr and

85Kr and the 511.9 keV peak of

106Ru.

4. Random Summing

A problem at high count rates is random summing of two main lines in aspectrum. This can lead to two problems, creation of additional lines in thespectrum (which will not normally be in the nuclide library) and loss of countsfrom the peaks concerned, which leads to undervaluation of these isotopes.This type of interference can be reduced by counting at a lower count rate, byeither using a smaller mass of sample or counting at a greater distance fromthe detector.

5. True Coincident Summing

With close counting geometries, true coincident summing can be significant,any gamma rays emitted in cascade from an isotope could in principle bedetected simultaneously and produce a sum peak. With a reverse electrodeHPGe detector which will be sensitive to low energy X-rays, coincidentsumming peaks can be seen between gamma rays and X-rays for internaltransition and electron capture decays. Again there will be extra peaksproduced which will not appear in nuclide libraries and consequent loss ofcounts from the summing peaks. This interference can only be reduced byreducing the solid angle presented by the source to the detector and somaking it unlikely that two radiations emitted at the same time from a singleatom or nucleus are both detected. This is usually accomplished by movingthe source to at least 10 cm from the detector.


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6. Gamma Interference

The final form of interference is due to gamma rays of similar energy fromdifferent isotopes being misidentified or of identical energy and the peak areanot being correctly split between the two isotopes. This type of interferencecan be best overcome by a matrix inversion technique that takes into accountall the peaks in the spectrum and all possible isotope energies. Usually aunique combination of isotopes can be determined.

This process is sometimes simplified by calculating the activity of one isotopefrom only the non-interfered peaks and then back calculating the expectedcontribution to the interfered peak, the remaining area of the interfered peakmay then be associated with another isotope.

This type of direct interference is quite common and some well-knownexamples are listed in Table 3.

Energy Isotope Energy Isotope Energy Isotope

59.54 Am-241 59.32 Ta-182122.06 Co-57 121.1 Se-75 121.82 Kr-90

129.7 Kr-77 129.85 Kr-85m

136.47 Co-57 135.9 Se-75 136.0 Kr-79197.15 O-19 196.32 Kr-88 196.56 Xe-129

m

243.4 Xe-125 242.56 Xe-138249.3 Kr-90 249.79 Xe-135279.5 Se-75 279.18 Hg-203344.7 Kr-79 345.03 Kr-89 344.27 Eu-152402.58 Kr-87 400.70 Se-75511.01 Annihilation 513.99 Sr-85 513.99 Kr-85511.01 Annihilation 511.80 Ru-106563.25 Cs-134 564.24 Sb-122795.85 Cs-134 795.07 Ac-228810.75 Co-58 810.46 Eu-152 811.77 Eu-156

832.0 Kr-79 834.83 Kr-88 834.8 Mn-541115.52 Zn-65 1115.1 Kr-791120.49 Sc-46 1120.28 Bi-2141293.64 Ar-41 1291.56 Fe-591332.51 Co-60 1332.1 Kr-79

Table 3 Table of some Interfering Isotopes and their Gamma Ray

Energies


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Selecting a Counting Geometry

1. Ideally every detector should be efficiency calibrated with a sourcemade of the same material and in the same shape as the samples andcounted at the same distance from the detector. Unless the samplesare in liquid form this requirement is rarely met. Compromises mustthen be made and suitable corrections applied for any variationbetween the sample and the calibration source.

The most common radioactivity standards that are readily availableare in the form of standardised liquids or gases and reference pointsources deposited on Mylar film. With care suitable gas and liquidcalibration standards may be prepared from the standardised gases orsolution, and spiked soil, and or other granular calibration sourcesmay be generated for volume sources of variable density.

A common procedure for efficiency calibration is to prepare sourcesfor the commonly used geometries in the laboratory (e.g. 10 ml vial ofliquid or 1000 ml Marinelli beaker) and determine the efficiencyfunction for these frequently used geometries. The point sources arealso often used to generate efficiency curves at various distances fromthe detector (say 10 cm and 30 cm), which can then be used as thebasis for determining the activity of less often used countinggeometries.

2. Before choosing any particular counting geometry the accuracyrequired for the analysis must be decided. If it is only necessary toidentify the isotopes present, then any geometry would be provided thespectrometers count rate limit was not exceeded. Conversely if anaccuracy of much better than 5% is required then there would probablyhave to be an extensive check on the whole system to ensure thatsuch accuracy was possible. If an accuracy of 5% is adequate thenmost well set up systems should satisfy this criterion if an identicalgeometry to the sample has been calibrated.

3. The next thing to consider is the sample count rate; is the sample ofsuch a low activity that it needs to be counted in a low backgroundshield, or is it so active that it must be counted at some distance fromthe detector? At both ends of the count rate scale the accuracyavailable will most probably be reduced. At low count rates theaccuracy will be limited by the background count rate and at highcount rates the pile up correction will be limiting.

These two problems can be eased by ensuring that a suitable mass orvolume of sample is counted. For a low specific activity source, suchas an environmental sample, a large volume could be counted in aMarinelli beaker. Conversely for a high activity source a small aliquotcould be counted, assuming the sample is homogeneous and arepresentative sample can be taken.


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Calculation o f Sample Activi ty

In modern computer based gamma spectrometers most of the calculationsnecessary to determine a sample activity are carried out automatically and theresults printed out for the user. However some final corrections to theseresults may still be necessary for non-standard sources or counting geometriesand for correction for true coincidence summing.

The complete analysis process can be briefly summarised:

(i) Locate all the peaks.(ii) Determine the peak areas (counts).(iii) Determine the peak positions and calculate their energies.(iv) Calculate for count rate for each peak.(v) Correct for random summing (if available).(vi) Subtract known background peak count rates (if available).(vii) Calculate the gamma ray emission rates using the detector efficiency

table or function.(viii) Match the peaks found with the nuclide library to identify all possible

isotopes present.(ix) Correct the emission rates for radioactive decay and gamma ray

abundance.(x) Calculate the activity of the provisionally identified isotopes.(xi) Check whether the identified isotopes are realistic and reject those

that are unlikely.

The gamma spectrometer analysis computer will handle most of thecalculation with minimal user intervention with the possible exception of thelast stage where some knowledge and experience will be needed. The abovesteps are now described in more detail.

The first step in the analysis after the spectrum has been collected and storedis to locate all the peaks in the spectrum and to determine their areas. Thiscan be a two-stage process but with some programs it is done in a single pass.The peak location process can be complex but basically it consists ofsmoothing the background continuum and finding significant deviations abovethe background.

This can be done either by looking at the second difference of the spectrum orby convoluting the spectrum with a peak shaped function. Whatever method isused the peak area is normally determined by fitting a combination of aGaussian shaped peak and a smooth background function. Some programsincorporate extra refinements to account for low and high energy tailing.

Once the peak positions and peak areas are determined the peak energiesand peak count rates can be calculated using the energy calibration and thesystem live time. If the system has a method of correcting for randomsumming (pulse pile-up correction) this can be done now. The correction isapplied through a function derived either from the system total count rate orthe system dead time. The correction is normally a simple multiplier used onall the peak count rates.


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For low sample count rates a correction is applied to subtract knownbackground peak count rates from the measured peaks in the system. Insome systems the background activity may be subtracted from the calculatedactivity at the end of the analysis. The former method is preferred to retain thesystem counting statistics.

The next stage for all samples is to determine the sample gamma rayemission rate from all the peaks. This is done by dividing the net peak countrate by the detector efficiency at the relevant peak energy. The efficiency maybe determined either directly from an algebraic function describing the detectorenergy-efficiency response, or from interpolation of an energy-efficiency look-up table.

The detector efficiency table or function is usually measured using either singlegamma ray emitting isotopes with no coincident summing or in a geometrysuch that coincident summing corrections are small. Any corrections toisotopes for coincident summing must be carried out manually at the end ofthe analysis.

The gamma ray energies and emission rates from the sample are now known;these must be correlated with possible isotopes. This is usually done bycomparing the measured gamma ray energies one by one with the gammarays listed in isotope name in a nuclide library; a nuclide identification program.Any gamma ray in the spectrum matching in energy with a gamma ray in thelibrary within a specified energy difference range is provisionally presumed tohave been emitted by that isotope. This is carried out for all the gamma raysin the spectrum. Some gamma rays in the spectrum may not match with thelibrary; they may be due to summing peaks or escape peaks or isotopes thatare not in the library. Some gamma rays may match more than one isotope.

When the gamma rays have been provisionally identified corrections are madefor the gamma ray abundance (emission per decay) and for half-life dependentdecay processes. There are two major processes that occur in most samplesand the corrections are usually carried out in the computer. The two decaycorrections that are calculated are:

1. Decay during the sample counting time.

2. Decay during the period between sample collection and the start of thesample count time.

There is a third decay process that has sometimes to be applied and that is fordecay during the sample collection time, e.g. for decay to a short-lived isotopethat is being collected on a filter over a long collection time. A diagram ofthese corrections is given in Figure 13.


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Figure 13 Decay Corrections to Sample Activit y

The calculated activity for every measured gamma ray has now beencalculated. Some gamma rays may have been provisionally allocated to morethan one isotope as the isotopes may emit gamma rays of similar energy. Thisinterference effect can be resolved in two ways. The most elegant method isto construct a matrix of all the measured gamma rays and all the possibleisotopes during the nuclide identification process and then to solve the matrixto allocate the relevant components of the interfering peaks to each isotope.

The alternative is to determine the mean activity of each isotope using the non-interfering peaks and from this mean calculate the relevant contribution for theinterfered peaks; the remainder may then be allocated to alternative isotopes.

The provisional allocation of peaks to isotopes and the consequent activitiesare then tested to see if they are probable. Two tests are commonly used, thefirst checks if the cooling time was large compared with the isotope half-life; ifthe ratio is larger than some preset value the provisional allocation is rejectedas unlikely. The second test checks that for those isotopes that emit severalgamma rays most of the gamma rays have been identified. This check mayjust be a numerical check e.g. more than 75% of all lines are present, or theprogram may use the gamma ray abundance and look for several rayscontributing a total of more than a specified fraction of the total number of

gamma rays emitted. This test will never reject a single gamma ray emittingisotope but will reject those isotopes identified with only one or a few gammarays that are provisionally allocated due to coincident or close energy gammarays.


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Once all the provisional allocations have been confirmed or rejected theprogram may compute a final activity for each isotope by one of three ways:

Either (1) The activity is determined from one specified gamma ray.

or (2) The best-fit activity for all isotopes is calculated using thematrix inversion method.

or (3) The weighted mean is calculated from all the lines identifiedfor each isotope.

At the end of this process some correction may have to be applied manuallydependant upon the precision of the required answer and the precisecalculational route.

If no pulse pile-up corrections were made this correction will have to becalculated and applied to all isotopes. If the efficiency curve is derived fromessentially coincidence free isotopes then corrections will have to be appliedon an isotope-by-isotope, gamma ray by gamma ray basis to correct for thecoincident summing. If the sample was counted at a distance or more than 10cm from the detector and the accuracy precision is 5% or less this correctionwill not be necessary.

If the sample density and shape does not conform to the calibration sourcethen corrections will have to be made for self-absorption.

As described earlier in this section most computer based gammaspectrometers are able to test the provisional allocation of gamma rays tospecific isotopes. The tests, if carried out with appropriate criteria, willeliminate most misidentified isotopes. However there are two other tests thatare important. The first is for programmes that do not correct or test forgamma ray interference. If one gamma ray could be associated with two ormore isotopes check that the peak count rate is not fully accounted for by oneisotope. The second and most important check is the experience and skill ofthe operator; are the samples being counted likely to contain the identifiedisotopes? In some cases isotopes can simply be ruled out as unlikely to bepresent in that type of sample. If a sample of steel is being counted foractivation products, it is unlikely to contain fission gases, etc.

Reference Material

Radiation Detection and Measurement, Knoll, 2nd

Edition, Wiley.

Nuclear and Radiochemistry, Friedlander, 3rd

Edition, Wiley-Interscience.

gamma spectroscopy principles

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