Electronics for RadiationElectronics for Radiation Detection Systems

• Introduction• PreamplifierPreamplifier• Amplifier

P l H i h A l• Pulse Height Analyzers• Display Mode• Cathode Ray Tubes

Introduction

Preamplifier

• To amplify the relatively small signal from the detectors

• To match the impedance levels between the detector and subsequent componentsdetector and subsequent components

• To shape the signal pulse for optimal subsequent processingsubsequent processing

A few points• The output from the preamplifier is V=Vo exp(-t/RC)The output from the preamplifier is V Vo exp(-t/RC)

where Vo =Q/C and RC is the time constant, typical 20-200 µsec for nuclear detectors.

• The amplification for scintillation detectors is small (5-20) because the signals from the detectors have already been amplified by photo-multiply tubes (105-1010).amplified by photo multiply tubes (10 10 ).

• Higher amplification is required for semiconductor detectors (103-104) due to small detector signals.

• Preamplifier is located as close as possible to the detector to maximise the signal to noise ratio (often in single unit).

AmplifiersT lif th till ll i l f th• To amplify the still small signals from the preamplifier (1-1000).

• To reshape the slow decaying pulse from preamplifier into a narrow one (for high count rate and increasing the S/N ratio etc,).

• Requirements for shaping: preserve the q p g pinput signal information such as pulse height and rise time.g

RC Shaping

• Differentiate circuit: – The output is a rapid rising pulse with decay constant τd

=RC which is smaller than that in preamplifier. – The amplitude of output is proportional to the rising

portion of the input and insensitive to the tailsportion of the input and insensitive to the tails. – It discriminates against low frequency noise.

• Integration circuit:Integration circuit: – output pulse rises with time constant. V=Vo(1-e-t/RC).– It discriminates against high frequency noise.g g q y

RC Shaping (continued)

• Differentiate plus Integration circuit:– The output amplitude is determined by the p p y

input– Time constant is shortened (0.25-5 µsec for

scintillation and semiconductor detectors, in contrast to 50-500 µsec in the preamplifier).

– Only one polarity (except for some small negative overshoot at the end)

RC Shaping (continued)

• Double differentiation plus integration circuit– Output is bipolarp p– Shorter rising time and longer total duration

than unipolar output– Preferred for high counting rate

Baseline Shift and Pulse Pileup

• Baseline Shift is caused by the negative component of the output (at the end of the p p (pulse)

• Pulse Pileup is caused by high countingPulse Pileup is caused by high counting rates that they fall on top of each other.

A few Points

• Baseline shift and pulse pileup are caused by high counting ratesB h bl b d d b h i i• Both problems can be reduced by shortening time constant but also reduce the energy resolution and S/N ratio.

• Double differentiate bipolar amplifier and short timeDouble differentiate bipolar amplifier and short time constant (0.025-0.5 u sec) are commonly used for NaI(Tl) detectors

i l d l i (0 8 ) f• Unipolar and longer time constant (0.5-8 u sec) for semiconductor detectors (achieve high energy resolution).

Pulse-Height Analyzers

• Basic Functions• Single Channel AnalyzersSingle Channel Analyzers• Time Methods

M l i h l A l• Multi-channel Analyzers

Basic Function• The amplitude of output signal is• The amplitude of output signal is

proportional to the energy of the radiation event detectedevent detected

• Selective counting of those pulses within i li d l d i i fcertain amplitude resulted in counting of

selective energy range• A certain energy range or interval is called

energy channel

Single Channel Analyzers• A kind of differential discriminator and responds only when pulse p y p

height is greater/lesser than the threshold level (adjustable). Its response is in the form of slandered logic signal.

• Composed of three parts: Lower Level Discriminator (LLD) UpperComposed of three parts: Lower Level Discriminator (LLD), Upper Level Discriminator (ULD) and Anticoincidence.

• With detectors where the output is proportional to energy, the SCA i d t t b h i ll fi dis used to measure energy spectra by choosing a small, fixed window and systematically sweeping the window across the full pulse height range. The relative no. of counts per unit time at each l l h b l d i hi f hlevel can then be plotted to give a histogram of the spectrum.

i dwindow

Single Channel Analyser: Operation

SCA t i th d• SCA can operate in three mode:– Normal or differential mode: lower level and upper level can

be adjusted independentlybe adjusted independently.

– Window mode: lower level and window width is set. , most W dow ode owe eve d w dow w d s se . , ossuitable for spectrum analysis because of a certain resolution (window) width.

– Integral Mode: A single channel analyser without ULD (simple discriminator with adjustable lower level) The number of signalsdiscriminator with adjustable lower level). The number of signals which pass is the integral of all the pulses from the threshold to the maximum limit of the SCA.

Single Channel Analyser: threshold level • The stability and linearity of the SCA threshold is an important factor.The stability and linearity of the SCA threshold is an important factor.• The degree to which the threshold control and the actual threshold

correspond to each other is referred to as integral linearity Li.E ll i t t i th diff ti l li it L hi h i f th• Equally important is the differential linearity Ld which is a measure of the constancy of the window width as the lower level is changed.

V

max

max100VV

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VL 100

Working range

Timing Method

• Determine the timing of radiation event is important in Nuclear radiation detection.

• An extension of the normal SCA is the timing SCA which also• An extension of the normal SCA is the timing SCA which also include circuits for correcting walk in the generation of the logic signal.

• There are a number of timing methods available but two of those are often used in nuclear detection: leading-edge and zero-crossing.

• Leading-edge uses the rising portion of the input pulse to trigger theLeading edge uses the rising portion of the input pulse to trigger the lower level discriminator which depends on the pulse amplitude (suffer certain amount of inaccuracy ~5 to 50 nsec for NaI (Tl)).

• Zero crossing requires bipolar pulses and is more accurate (4 nsec• Zero-crossing requires bipolar pulses and is more accurate (4 nsec for NaI(Tl)).

Multichannel AnalysersTh i i l f h l M l i h l A l (MCA) i• The principle of the popular Multichannel Analyser (MCA) is different from the single channel analyser.

• A sophisticated device which sort s out incoming pulses according to p g p gpulse height and keep count of the number at each height in a multichannel memory. The contents of each channel can then be displayed on a screen to give a pulse height spectrum.p y g p g p

• Simultaneous recording of multiple energy radiations.• The centre of the MCA is the analog-to-digital converter (ADC). The

MCA works by digitizing the amplitude of the incoming pulse with an analog -to- digital converter (ADC).

• Total no. of channels into which the voltage range is digitized isTotal no. of channels into which the voltage range is digitized is known as conversion gain. Range of conversion gain is 128-16K.

• A memory is required for the sorting of energy channels (energy t )ranges, energy spectrum).

Analog-to-Digital Converter

• Two types of ADC are used in nuclear medicine for MCA and the interface between scintillation cameras and computers: Wilkinson or Ramp converter and successivecomputers: Wilkinson or Ramp converter and successive approximation

• Both require time for the conversion which could be a q“bottle neck” for the time resolution but is not a major problem for nuclear medicine application

• Both of the converters use binary number representation• Both of the converters use binary number representation which means that the more bits the more accurate but requires more time and memory.

Ramp ADC

• RC circuitry and clock oscillator• Discharging time proportional to the amplitudeDischarging time proportional to the amplitude

of the input pulse (radiation energy)• Clock oscillator produces pulse train that areClock oscillator produces pulse train that are

counted in a counting circuit• The number of the clock pulses counted areThe number of the clock pulses counted are

proportional to the discharging time which in turn proportional the radiation energy).p p gy)

Successive Approximation

• The input pulse is compared with one-half of the full scale

• The comparison voltage is then either increased or decreased by one half of itsincreased or decreased by one half of its initial level depending on whether the pulse amplitude did or did not exceed the initialamplitude did or did not exceed the initial level.

• The process is repeated for several steps• The process is repeated for several steps.

Time to Amplitude Converter

Scalers and Timers

• A device that only counts pulses is called a scaler

• An auxiliary device that controls the scaler counting time is called timercounting time is called timer.

Analog Ratemeters

• A analog ratemeter is used to determine the average number of events occurring per unit time. The average is determined continuously rather than over discrete counting time

• Linear vs logarithmic ratemeters: V0=knQRp vs V0=klog(nQRp) - wider range of counting rate

• Ratemeter responds to the rate change has a time constant which can be adjusted (change the

i )capacitor)

Coincidence Unit