EEE4106Z Radiation Interactions & Detection 2. Radiation Detection Dr. Steve Peterson 5.14 RW James Department of Physics University of Cape Town steve.peterson@uct.ac.za 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 EEE4106Z :: Radiation Interactions & Detection 2 / 46 Dr. Steve Peterson
References J. S. Lilley, Nuclear Physics: Principles and Applications, Wiley 2001 - [Ch 6] K. S. Krane, Introductory Nuclear Physics, Wiley 1988 - [Ch 7] G. F. Knoll, Radiation Detection and Measurement (4th ed.), Wiley 2010 - [Ch 2, 10] EEE4106Z :: Radiation Interactions & Detection 3 / 46 Dr. Steve Peterson
Radiation Detection Most detectors of nuclear radiation follow similar characteristics: Radiation enters the detector It interacts with the atoms of the detector material (losing some or all of its energy) A large number of relatively low-energy electrons are released from their atomic orbits These electrons are collected and formed into voltage or current pulses for analysis by electronic circuitry We will discuss general characteristics of detectors and some commonly-used detector types. EEE4106Z :: Radiation Interactions & Detection 4 / 46 Dr. Steve Peterson
Radiation Detection Neutral radiation must undergo some sort of reaction in the detector producing charges particles, which in turn ionize and excite the detector atoms. Note: characteristic distance of penetration or average path length (range or mean free path) in solids for typical energy radiations in each category EEE4106Z :: Radiation Interactions & Detection 5 / 46 Dr. Steve Peterson
Detector Response Most detectors are also capable of providing some information on the energy of the radiation, using the fact that the amount of ionization produced by radiation in a detector is proportional to the energy it loses in the sensitive volume. The output signal of an electrical detectors is in the form of a current pulse. The amount of ionization is then reflected in the electrical charge contained in this signal, i.e., the integral of the pulse with respect to time. The relation between the radiation energy and the total charge or pulse height of the output signal is referred to as the response of the detector. EEE4106Z :: Radiation Interactions & Detection 6 / 46 Dr. Steve Peterson
Response Function The response function of a detector is the spectrum of pulse heights observed when it is bombarded by a mono-energetic beam of a given radiation. The ideal case would be a Dirac delta function, i.e. for a fixed incident energy the output signal has a single, fixed amplitude. In reality, a Gaussian or even more complicated response spectrum results. Factors that might affect the detector response Detector Material Detector Size Detector Geometry Radiation Type Radiation Energy EEE4106Z :: Radiation Interactions & Detection 7 / 46 Dr. Steve Peterson
Response Function The response functions of two different detectors for 661 kev gamma rays. (a) Germanium (b) Organic scintillator EEE4106Z :: Radiation Interactions & Detection 8 / 46 Dr. Steve Peterson
Response Function Examples of response functions for detectors with relatively good resolution and relatively poor resolution EEE4106Z :: Radiation Interactions & Detection 9 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Like neutrons, gamma-ray are uncharged and create no direct ionization or excitation of material through which it passes. The detection of gamma rays is critically dependent on the fast electrons created in the gamma-ray interactions to provide any clue to the nature of the incident photon. For a detector to effectively serve as a gamma-ray spectrometer, it must carry out two distinct functions: It must be a medium in which incident gamma rays will interact to produce one or more fast electrons It must function as a conventional detector for these secondary electrons EEE4106Z :: Radiation Interactions & Detection 10 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays NOTE: We will assume that our detector is large enough to capture all secondary electrons. Electrons of a few MeV in a solid detector will only travel a few mm. On the other hand, a 1 MeV electron can travel several meters in a gas at STP, thus removing gas-filled detectors from our discussion. The three main gamma interaction mechanisms all have significance in gamma-ray spectroscopy: Photoelectric absorption Compton scattering Pair production EEE4106Z :: Radiation Interactions & Detection 11 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays In the photoelectric absorption process, the photon energy gets carried off by the photo-electron, together with one or more low-energy electrons. If none of the electrons escape the detector, the sum of the electron energies must equal the original energy of the photon, making it the ideal process for measuring the gamma-ray energy. EEE4106Z :: Radiation Interactions & Detection 12 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays A Compton scattering interaction produces a recoil electron and a scattered gamma-ray photon, with a division of the energy. The energy of the scattered gamma ray is: E γ = E γ 1 + (E γ /mc 2 )(1 cos θ) The kinetic energy of the recoil electron is: E e = E γ (E γ /mc 2 )(1 cos θ) 1 + (E γ /mc 2 )(1 cos θ) (1) (2) EEE4106Z :: Radiation Interactions & Detection 13 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays The two extreme cases are: A grazing angle scatter (θ 0), where E γ = E γ and E e = 0. A head-on collision (θ = π), where the gamma-ray is backscattered and the maximum energy is transferred to the electron, which is (E e ) max = E γ (2E γ /mc 2 ) 1 + (2E γ /mc 2 ) (3) The gap between the maximum Compton recoil energy and the incident gamma-ray energy is given by E C = E γ (E e ) max = E γ 1 + (2E γ /mc 2 ) (4) EEE4106Z :: Radiation Interactions & Detection 14 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays For any one specific gamma-ray energy, the electron energy distribution has this general shape EEE4106Z :: Radiation Interactions & Detection 15 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays The pair production process consists of converting the incident gamma-ray photon into electron and positron kinetic energies T + T + = E γ 2mc 2 (5) For typical energies, the electron and positron travel a few mm at most before losing all their kinetic energies (minus two annihilation photons of energy mc 2 =0.511 MeV), creating a peak located at E γ 2mc 2, also called the double escape peak. EEE4106Z :: Radiation Interactions & Detection 16 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Small Detectors EEE4106Z :: Radiation Interactions & Detection 17 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Small Detectors EEE4106Z :: Radiation Interactions & Detection 18 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Very Large Detectors EEE4106Z :: Radiation Interactions & Detection 19 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Very Large Detectors EEE4106Z :: Radiation Interactions & Detection 20 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Intermediate Size Detectors EEE4106Z :: Radiation Interactions & Detection 21 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Intermediate Size Detectors EEE4106Z :: Radiation Interactions & Detection 22 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays 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 EEE4106Z :: Radiation Interactions & Detection 23 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Influence of surrounding materials on detector response EEE4106Z :: Radiation Interactions & Detection 24 / 46 Dr. Steve Peterson
Detector Response for Gamma Rays Influence of surrounding materials on detector response EEE4106Z :: Radiation Interactions & Detection 25 / 46 Dr. Steve Peterson
Gas-filled Detectors Several of the oldest and most widely used types of radiation detectors are based on the effects produced when a charged particle passes through a gas. The primary modes of interaction involve ionization and excitation of gas molecules along the particle track. The detectors that we will cover (ionization chambers, proportional counters, Geiger-Muller counters) all derive, in somewhat different ways, an electronic output signal that originates with the ion pairs formed within the gas filling the detector. EEE4106Z :: Radiation Interactions & Detection 26 / 46 Dr. Steve Peterson
Ionization Chambers Ionization chambers in principle are the simplest of all gas-filled detectors. The basic version consists of two electrodes forming a parallel-plate capacitor C between which a voltage V is applied, normally through a large bias resistor R. The electric field keeps the ion pairs from recombining, splitting the ion pairs, electrons to the anode and positive ions to the cathode. EEE4106Z :: Radiation Interactions & Detection 27 / 46 Dr. Steve Peterson
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 10 4 ions and electrons. For a medium-sized chamber, say 10 10 cm with a plate separation of 1 cm, the capacitance is 8.9 10 12 F and the resulting voltage pulse is about (3 10 4 ions)(1.6 10 19 C/ion) 8.9 10 12 F 0.5mV This is a rather small signal, which must be considerably amplified (by a factor of roughly 10 4 ) before it can be analyzed by standard electronics. EEE4106Z :: Radiation Interactions & Detection 28 / 46 Dr. Steve Peterson
Ionization Chambers The amplitude of the signal is proportional to the number of ions formed (and thus the energy deposited by the radiation), and is independent of the voltage between the plates. The applied voltage does determine the speed at which the electron ion 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 to travel across a 1-cm chamber. (Electrons travel about 1000 times faster) This is an exceedingly long time by standards of nuclear counting (a weak 1 µci source gives on average one decay every 30 µsec), thus the ion chamber is of no use in counting individual pulses. EEE4106Z :: Radiation Interactions & Detection 29 / 46 Dr. Steve Peterson
Ionization Chambers Ionization chambers are used extensively as radiation monitors and in radiation dosimetry EEE4106Z :: Radiation Interactions & Detection 30 / 46 Dr. Steve Peterson
Proportional Counters To use a gas-filled detector to observe individual pulses, we must provide considerable amplification, typically achieved by increasing the voltage, in excess of 1000 V. The larger electric field is able to accelerate the electrons that result from the ionization process; rather than drifting slowly toward the anode. The accelerated electrons can acquire enough energy to make inelastic collisions and even create new ionized atoms (and new electrons). The rapid amplification through production of secondary ionizations is called a Townsend avalanche. EEE4106Z :: Radiation Interactions & Detection 31 / 46 Dr. Steve Peterson
Proportional Counters Even though there is a large number (10 3-10 5 ) of secondary events, the chamber is always operated such that the number of secondary events is proportional to the number of primary events. EEE4106Z :: Radiation Interactions & Detection 32 / 46 Dr. Steve Peterson
Geiger-Muller Counters The two curves correspond to different amounts of energy deposited in gas. If the electric field is increased to even larger values, secondary avalanches can occur. These are the result of ionized electrons exciting neighbouring atoms, which de-excite to produce photons which in turn ionize other atoms, until the entire volume is participating. This region of operation is called the Geiger-Muller region. EEE4106Z :: Radiation Interactions & Detection 33 / 46 Dr. Steve Peterson
Geiger-Muller Counters The amplification factor can be as large as 10 10. Because the entire tube is participating, there is no information about the energy of the original radiation - all incident radiations produce identical signals. The output signal is of the order of 1 V, so no further amplification is usually required. The collection time is of the order of 10 6, but the positive ions do not move very far from the avalanche region. The cycle would be completed after the positive ions have drifted to the cathode and become neutralized (which takes 10 4 10 3 ), which strike with enough energy to release electrons, which restarted the whole process again. In order to prevent this, a quenching gas is added to the tube, which absorbs the free electrons produced when the positive ions hit the cathode. EEE4106Z :: Radiation Interactions & Detection 34 / 46 Dr. Steve Peterson
Radiation Detection Detector Response Gas-Filled Scintillation Semiconductor Neutron Geiger-Muller Counters Geiger counters are popular as portable radiation monitoring instruments. EEE4106Z :: Radiation Interactions & Detection 35 / 46 Dr. Steve Peterson
Scintillation Detectors The scintillation detector is one of the most widely used particle detection devices in nuclear and particle physics today. It makes use of the fact that certain materials when struck by a nuclear particle or radiation, emit a small flash of light, i.e. a scintillation. When coupled to an amplifying device such as a photomultiplier, these scintillations can be converted into electrical pulses. EEE4106Z :: Radiation Interactions & Detection 36 / 46 Dr. Steve Peterson
Scintillation Detectors The complete scintillation detection process is: 1. The incident radiation enters the detector, producing a large number of interactions, exciting atoms within the detector. 2. The excited states rapidly emit visible (or near-visible) light; the material is said to fluoresce. 3. The light strikes a photosensitive surface, releasing at most one photoelectron per photon 4. The secondary electrons are then multiplied, accelerated, and formed into the output pulse in the photomultiplier tube (PMT) EEE4106Z :: Radiation Interactions & Detection 37 / 46 Dr. Steve Peterson
Scintillation Detectors In a PMT, the gain per stage may be as high as five, giving an overall amplification for a 10-stage tube of 5 10 or about 10 7 EEE4106Z :: Radiation Interactions & Detection 38 / 46 Dr. Steve Peterson
General Characteristics of Scintillation Detectors In general, the scintillator signal is capable of providing a variety of information. Among its most outstanding features are: 1. Sensitivity to Energy. Both scintillators and photomultipliers are linear with respect to the energy deposited, thus the amplitude of the final electrical signal will also be proportional to this energy. 2. Fast Time Response. The response and recovery times of scintillation detectors are short relative to other types of detectors. This allows for timing information to be obtained and to accept higher count rates. 3. Pulse Shape Discrimination. With certain scintillators, it is possible to distinguish between different types of particles by analyzing the shape of the emitted light pulses. EEE4106Z :: Radiation Interactions & Detection 39 / 46 Dr. Steve Peterson
Semiconductor Detectors Semiconductor detectors are widely used in research and industry where an accurate measurement of energy is needed. The basic operating principle of semiconductor detectors is analogous to gas ionization devices. Instead of a gas, however, the medium is now a solid semiconductor material. The passage of ionizing radiation creates electron-hole pairs as in an inorganic scintillator (versus electron-ion pairs as in gas ionization) The electrons in the conduction band are mobile and, under the influence of an applied electric field, move through the crystal at a speed determined by their mobility. Vacancies or holes in the valence band also move, but in the opposite direction. The movement of electrons and holes in a semiconductor constitutes a current and the result is a pulse proportional to the energy deposited in the crystal. EEE4106Z :: Radiation Interactions & Detection 40 / 46 Dr. Steve Peterson
Semiconductor Detectors Advantages of semiconductor detectors Increase energy resolution Greater density and greater stopping power than gas detectors Compact in size Very fast response times Variable effective thickness Disadvantages of semiconductor detectors Limited to relatively small sizes Relatively high susceptibility to performance degradation from radiation-induced damage Generally require cooling to low temperatures before they can be operated EEE4106Z :: Radiation Interactions & Detection 41 / 46 Dr. Steve Peterson
Semiconductor Detectors Two types of semiconductor detectors are in common use. One is called a p-n junction detectors and the other an intrinsic detector. p-n junction detector This type of detector is a diode, formed at the boundary between two different types of semiconductors, typically made of silicon Intrinsic detector This type of detector is made from a single crystal of hyperpure (intrinsic) material, typically made of germanium. EEE4106Z :: Radiation Interactions & Detection 42 / 46 Dr. Steve Peterson
Neutron Detectors Neutrons do not produce direct ionization events, so neutron detectors must be based on detecting the secondary events produced by nuclear reactions. For slow and thermal neutrons, detectors based on the (n,p) and (n,α) reactions provide a direct means for observing neutrons from the signal left by the energetic secondaries resulting from the reactions. For fast neutrons, nuclear scattering from light charged particles can give enough energy to a recoiling nucleus for detection. EEE4106Z :: Radiation Interactions & Detection 43 / 46 Dr. Steve Peterson
Neutron Detectors For slow and thermal neutrons, the isotope 10 B is commonly used, either using BF 3 gas in an ionization chamber or proportional counter, or lining a detector with boron metal or other boron compound. Inside a proportional counter, BF 3 is both the target for the nuclear reaction and the counter fill gas. The reaction is 10 B + n 7 Li + α where 7 Li is preferentially left in an excited state with energy 0.48 MeV, reducing the sum of the α and 7 Li kinetic energies to 2.31 MeV. These charged particles cause ionization in the detector gas, which gives rise to an output signal. EEE4106Z :: Radiation Interactions & Detection 44 / 46 Dr. Steve Peterson
Neutron Detectors The reaction cross section for 10 B(n,α) 7 Li has a 1/v dependence. Some other commonly used neutron detection reaction are shown. EEE4106Z :: Radiation Interactions & Detection 45 / 46 Dr. Steve Peterson
Neutron Detectors It is more common to use a detector for fast neutrons consisting of a plastic or liquid organic scintillator. These materials have high hydrogen content and the signal comes from the energy of recoiling protons scattered by neutrons within the scintillator itself. The high density of hydrogen in the scintillator material and the larger interaction cross section means that these solid or liquid detectors are much more efficient for fast neutrons than any of the gas detectors based on neutron-induced reactions. EEE4106Z :: Radiation Interactions & Detection 46 / 46 Dr. Steve Peterson