eee4106z radiation interactions & detection - 2. radiation ... · radiation detection neutral...

Radiation Detection Detector Response Gas-Filled Scintillation Semiconductor Neutron

EEE4106Z

Radiation Interactions & Detection2. Radiation Detection

Dr. Steve Peterson

5.14 RW JamesDepartment of PhysicsUniversity of Cape [email protected]

May 06, 2015

EEE4106Z :: Radiation Interactions & Detection 1 / 46 Dr. Steve Peterson


Radiation Detection

Detector Response

Gas-Filled Detectors

Scintillation Detectors

Semiconductor Detectors

Neutron Detectors



References

I J. S. Lilley, Nuclear Physics: Principles and Applications, Wiley2001 - [Ch 6]

I K. S. Krane, Introductory Nuclear Physics, Wiley 1988 - [Ch 7]

I G. F. Knoll, Radiation Detection and Measurement (4th ed.),Wiley 2010 - [Ch 2, 10]



Radiation Detection

Most detectors of nuclear radiation follow similar characteristics:

I Radiation enters the detector

I It interacts with the atoms of the detector material (losingsome or all of its energy)

I A large number of relatively low-energy electrons are releasedfrom their atomic orbits

I These electrons are collected and formed into voltage or currentpulses for analysis by electronic circuitry

We will discuss general characteristics of detectors and somecommonly-used detector types.



Radiation Detection

Neutral radiation must undergo some sort of reaction in the detectorproducing charges particles, which in turn ionize and excite thedetector atoms.

Note: characteristic distance of penetration or average path length (range ormean free path) in solids for typical energy radiations in each category



Detector Response

Most detectors are also capable of providing some information onthe energy of the radiation, using the fact that the amount ofionization produced by radiation in a detector is proportional to theenergy it loses in the sensitive volume.

The output signal of an electrical detectors is in the form of acurrent pulse. The amount of ionization is then reflected in theelectrical charge contained in this signal, i.e., the integral of thepulse with respect to time.

The relation between the radiation energy and the total charge orpulse height of the output signal is referred to as the response of thedetector.



Response Function

The response function of a detector is the spectrum of pulse heightsobserved when it is bombarded by a mono-energetic beam of a givenradiation. The ideal case would be a Dirac delta function, i.e. for afixed incident energy the output signal has a single, fixed amplitude.In reality, a Gaussian or even more complicated response spectrumresults.

Factors that might affect the detector response

I Detector Material

I Detector Size

I Detector Geometry

I Radiation Type

I Radiation Energy



Response Function

The response functions of two different detectors for 661 keVgamma rays. (a) Germanium (b) Organic scintillator



Response Function

Examples of response functions for detectors with relatively goodresolution and relatively poor resolution



Detector Response for Gamma Rays

Like neutrons, gamma-ray are uncharged and create no directionization or excitation of material through which it passes. Thedetection of gamma rays is critically dependent on the fast electronscreated in the gamma-ray interactions to provide any clue to thenature of the incident photon.

For a detector to effectively serve as a gamma-ray spectrometer, itmust carry out two distinct functions:

I It must be a medium in which incident gamma rays will interactto produce one or more fast electrons

I It must function as a conventional detector for these secondaryelectrons




NOTE: We will assume that our detector is large enough to captureall secondary electrons. Electrons of a few MeV in a solid detectorwill only travel a few mm. On the other hand, a 1 MeV electron cantravel several meters in a gas at STP, thus removing gas-filleddetectors from our discussion.

The three main gamma interaction mechanisms all have significancein gamma-ray spectroscopy:

I Photoelectric absorption

I Compton scattering

I Pair production




In the photoelectric absorption process, the photon energy getscarried off by the photo-electron, together with one or morelow-energy electrons. If none of the electrons escape the detector,the sum of the electron energies must equal the original energy ofthe photon, making it the ideal process for measuring thegamma-ray energy.




A Compton scattering interaction produces a recoil electron and ascattered gamma-ray photon, with a division of the energy.

The energy of the scattered gamma ray is:

E ′γ =Eγ

1 + (Eγ/mc2)(1− cos θ)(1)

The kinetic energy of the recoil electron is:

Ee− = Eγ(Eγ/mc

2)(1− cos θ)

1 + (Eγ/mc2)(1− cos θ)(2)




The two extreme cases are:

I A grazing angle scatter (θ ≈ 0), where E ′γ = Eγ and Ee− = 0.

I A head-on collision (θ = π), where the gamma-ray isbackscattered and the maximum energy is transferred to theelectron, which is

(Ee−)max = Eγ(2Eγ/mc

2)

1 + (2Eγ/mc2)(3)

The gap between the maximum Compton recoil energy and theincident gamma-ray energy is given by

EC = Eγ − (Ee−)max =Eγ

1 + (2Eγ/mc2)(4)




For any one specific gamma-ray energy, the electron energydistribution has this general shape




The pair production process consists of converting the incidentgamma-ray photon into electron and positron kinetic energies

T− + T+ = Eγ − 2mc2 (5)

For typical energies, the electron and positron travel a few mm atmost before losing all their kinetic energies (minus two annihilationphotons of energy mc2=0.511 MeV), creating a peak located atEγ − 2mc2, also called the double escape peak.




”Small” Detectors




Very Large Detectors




Intermediate Size Detectors




Complications to the Response Function

1. Secondary electron escape

2. Bremsstrahlung escape

3. Characteristic x-ray escape

4. Secondary radiation created near the source

5. Effects of surrounding materials




Influence of surrounding materials on detector response



Gas-filled Detectors

Several of the oldest and most widely used types of radiationdetectors are based on the effects produced when a charged particlepasses through a gas. The primary modes of interaction involveionization and excitation of gas molecules along the particle track.

The detectors that we will cover (ionization chambers, proportionalcounters, Geiger-Muller counters) all derive, in somewhat differentways, an electronic output signal that originates with the ion pairsformed within the gas filling the detector.



Ionization Chambers

Ionization chambers in principle arethe simplest of all gas-filled detectors.

The basic version consists of twoelectrodes forming a parallel-platecapacitor C between which a voltageV is applied, normally through a largebias resistor R.

The electric field keeps the ion pairsfrom recombining, splitting the ionpairs, electrons to the anode andpositive ions to the cathode.



Ionization Chambers

In air, the average energy needed to produce an ion in about 34 eV;thus a 1-MeV radiation produces a maximum of 3× 104 ions andelectrons. For a medium-sized chamber, say 10× 10 cm with a plateseparation of 1 cm, the capacitance is 8.9× 10−12F and theresulting voltage pulse is about

(3× 104 ions)(1.6× 10−19C/ion)

8.9× 10−12F≈ 0.5mV

This is a rather small signal, which must be considerably amplified(by a factor of roughly 104) before it can be analyzed by standardelectronics.



Ionization Chambers

The amplitude of the signal is proportional to the number of ionsformed (and thus the energy deposited by the radiation), and isindependent of the voltage between the plates.

The applied voltage does determine the speed at which the electronion clouds drift to their respective electrodes. For a typical voltage(100 V), the ions move at about 1 m/s, taking roughly 0.01 s totravel across a 1-cm chamber. (Electrons travel about 1000 timesfaster)

This is an exceedingly long time by standards of nuclear counting (aweak 1 µCi source gives on average one decay every 30 µsec), thusthe ion chamber is of no use in counting individual pulses.



Ionization Chambers

Ionization chambers are used extensively as radiation monitors andin radiation dosimetry



Proportional Counters

To use a gas-filled detector to observe individual pulses, we mustprovide considerable amplification, typically achieved by increasingthe voltage, in excess of 1000 V.

The larger electric field is able to accelerate the electrons that resultfrom the ionization process; rather than drifting slowly toward theanode. The accelerated electrons can acquire enough energy tomake inelastic collisions and even create new ionized atoms (andnew electrons). The rapid amplification through production ofsecondary ionizations is called a Townsend avalanche.



Proportional Counters

Even though there is a large number (103 - 105) of secondaryevents, the chamber is always operated such that the number ofsecondary events is proportional to the number of primary events.



Geiger-Muller Counters

The two curves correspond to differentamounts of energy deposited in gas.

If the electric field isincreased to even largervalues, secondary avalanchescan occur. These are theresult of ionized electronsexciting neighbouring atoms,which de-excite to producephotons which in turn ionizeother atoms, until the entirevolume is participating. Thisregion of operation is calledthe Geiger-Muller region.




The amplification factor can be as large as 1010. Because the entiretube is participating, there is no information about the energy of theoriginal radiation - all incident radiations produce identical signals.

The output signal is of the order of 1 V, so no further amplificationis usually required. The collection time is of the order of 10−6, butthe positive ions do not move very far from the avalanche region.

The cycle would be completed after the positive ions have drifted tothe cathode and become neutralized (which takes 10−4 − 10−3),which strike with enough energy to release electrons, which restartedthe whole process again.

In order to prevent this, a quenching gas is added to the tube, whichabsorbs the free electrons produced when the positive ions hit thecathode.




Geiger counters are popular as portable radiation monitoringinstruments.




The scintillation detector is one of themost widely used particle detectiondevices in nuclear and particle physicstoday. It makes use of the fact thatcertain materials when struck by anuclear particle or radiation, emit a smallflash of light, i.e. a scintillation. Whencoupled to an amplifying device such asa photomultiplier, these scintillationscan be converted into electrical pulses.




The complete scintillation detection process is:

1. The incident radiation enters the detector, producing a largenumber of interactions, exciting atoms within the detector.

2. The excited states rapidly emit visible (or near-visible) light; thematerial is said to fluoresce.

3. The light strikes a photosensitive surface, releasing at most onephotoelectron per photon

4. The secondary electrons are then multiplied, accelerated, andformed into the output pulse in the photomultiplier tube (PMT)




In a PMT, the gain per stage may be as high as five, giving anoverall amplification for a 10-stage tube of 510 or about 107



General Characteristics of Scintillation Detectors

In general, the scintillator signal is capable of providing a variety ofinformation. Among its most outstanding features are:

1. Sensitivity to Energy. Both scintillators and photomultipliersare linear with respect to the energy deposited, thus theamplitude of the final electrical signal will also be proportionalto this energy.

2. Fast Time Response. The response and recovery times ofscintillation detectors are short relative to other types ofdetectors. This allows for timing information to be obtainedand to accept higher count rates.

3. Pulse Shape Discrimination. With certain scintillators, it ispossible to distinguish between different types of particles byanalyzing the shape of the emitted light pulses.




Semiconductor detectors are widely used in research and industrywhere an accurate measurement of energy is needed.

The basic operating principle of semiconductor detectors isanalogous to gas ionization devices. Instead of a gas, however, themedium is now a solid semiconductor material. The passage ofionizing radiation creates electron-hole pairs as in an inorganicscintillator (versus electron-ion pairs as in gas ionization)

The electrons in the conduction band are mobile and, under theinfluence of an applied electric field, move through the crystal at aspeed determined by their mobility. Vacancies or holes in the valenceband also move, but in the opposite direction. The movement ofelectrons and holes in a semiconductor constitutes a current and theresult is a pulse proportional to the energy deposited in the crystal.




Advantages of semiconductor detectors

I Increase energy resolution

I Greater density and greater stopping power than gas detectors

I Compact in size

I Very fast response times

I Variable effective thickness

Disadvantages of semiconductor detectors

I Limited to relatively small sizes

I Relatively high susceptibility to performance degradation fromradiation-induced damage

I Generally require cooling to low temperatures before they canbe operated




Two types of semiconductor detectors are in common use. One iscalled a p-n junction detectors and the other an intrinsic detector.

p-n junction detector

I This type of detector is a diode, formed at the boundarybetween two different types of semiconductors, typically madeof silicon

Intrinsic detector

I This type of detector is made from a single crystal of hyperpure(intrinsic) material, typically made of germanium.



Neutron Detectors

Neutrons do not produce direct ionization events, so neutrondetectors must be based on detecting the secondary events producedby nuclear reactions.

For slow and thermal neutrons, detectors based on the (n,p) and(n,α) reactions provide a direct means for observing neutrons fromthe signal left by the energetic secondaries resulting from thereactions.

For fast neutrons, nuclear scattering from light charged particles cangive enough energy to a recoiling nucleus for detection.



Neutron Detectors

For slow and thermal neutrons, the isotope 10B is commonly used,either using BF3 gas in an ionization chamber or proportionalcounter, or lining a detector with boron metal or other boroncompound.

Inside a proportional counter, BF3 is both the target for the nuclearreaction and the counter fill gas. The reaction is

10B + n→ 7Li∗ + α

where 7Li is preferentially left in an excited state with energy 0.48MeV, reducing the sum of the α and 7Li kinetic energies to 2.31MeV. These charged particles cause ionization in the detector gas,which gives rise to an output signal.



Neutron Detectors

The reaction cross section for 10B(n,α)7Li has a 1/v dependence.Some other commonly used neutron detection reaction are shown.



Neutron Detectors

It is more common to use a detector for fast neutrons consisting of aplastic or liquid organic scintillator. These materials have highhydrogen content and the signal comes from the energy of recoilingprotons scattered by neutrons within the scintillator itself.

The high density of hydrogen in the scintillator material and thelarger interaction cross section means that these solid or liquiddetectors are much more efficient for fast neutrons than any of thegas detectors based on neutron-induced reactions.


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