Dosimetry by Pulse-Mode Detectors II

1. Dosimetry by Pulse-Mode Detectors II Scintillation Dosimetry Semiconductor Detectors for Dosimetry

2. Scintillation Dosimetry: Introduction Many transparent substances, including certain solids, liquids, and gases, scintillate (i.e., emit flashes of visible light) as a result of the action of ionizing radiation By using a sensitive light detector such as a photomultiplier (PM) tube, the light emitted can be converted into an electrical signal

3. Introduction (cont.) A light photon incident on the photocathode of a PM tube may release an electron, which is then numerically amplified as much as ? 107 in passing through the dynode chain in the tube Either the output electrical pulses from numerous such events can be counted (with or without pulse-height analysis), or the output current can be measured

5. Introduction (cont.) Scintillators have been widely applied as detectors of ionizing radiation, especially in nuclear physics Very fast decay times, down to ~ 10-9 s, make organic liquid and plastic scintillators excellent choices for coincidence measurements with good time-resolution, and they can occupy whatever volume shape and size one wishes

6. Introduction (cont.) Scintillators, especially NaI(Tl), also have been used extensively for x- and ?-ray energy spectrometry but have been largely replaced by Si(Li) and Ge(Li) semiconductor detectors for best energy resolution However, scintillators continue to be widely used for those ?-ray spectrometry applications in which resolution is less critical, because of lower cost and the greater convenience of operating the detector at room temperature instead of inside a cryostat

7. Light Output Efficiency Only a very small part of the energy imparted to a scintillator appears as light; the rest is dissipated as heat In typical situations ~ 1 keV of energy is spent in the scintillator for the release of one electron from the PM tube’s photocathode However, the large gain available in the PM tube and external amplifiers still provides an adequate output signal

8. Efficiency (cont.) The light generated in a scintillator by a given imparted energy depends on the LET of the charged particles delivering the energy In typical organic scintillators, increasing the particle LET decreases the light output for a given energy imparted, as can be seen in the following diagram

10. Efficiency (cont.) The light response from electrons that spend their full track length in the scintillator is found to be proportional to their starting energy above about 125 keV For protons, the light output is only about 15% as great as for electrons at 1 MeV, rising to about 40% as great at 10 MeV The technique of pulse-shape discrimination allows the separation of dose components on the basis of particle LET

11. Efficiency (cont.) For dosimetry of ?-rays or electrons, either the PM-tube output should be measured as an electric current or the pulse-heights must be analyzed and calibrated in terms of dose, as discussed for proportional counters Simple counting of pulses without regard to their size is not a measure of the dose in a scintillator

12. Scintillator Types For most dosimetry applications where soft tissue is the dose-relevant material, organic plastic scintillators such as NE-102, organic liquids such as NE-213, and the organic crystals stilbene and anthracene are the most useful because they are made mostly of the low-atomic-number elements C and H Thus they do not overrespond to photons through the photoelectric effect, and the hydrogen content makes the (n, p) elastic-scattering interaction the main process for fast-neutron dose deposition, as it is in tissue

14. Light Collection and Measurement: Scintillator Enclosure A light reflector, optimally a thin layer of MgO powder, is useful to maximize light-collection efficiency from a scintillator If the scintillator has polished surfaces, all the light incident from the inside is reflected if the angle of incidence is greater than the critical angle The MgO reflector will recapture most of the light that escapes at smaller angles

15. Scintillator Enclosure (cont.) For small or thin scintillators (plastic ones may be as thin as ~ 20 ?m) one should keep in mind cavity-theory considerations Simplest dosimetric interpretation for indirectly ionizing radiation calls for surrounding the scintillator by a nonscintillating layer of the same composition, thick enough to provide CPE

16. Scintillator Enclosure (cont.) In the case of plastic scintillators a shell of Lucite will usually suffice, surrounding the thin reflector Outside of this an opaque covering such as aluminum foil is required to exclude ambient light NaI and CsI scintillators require hermetic seals, as they are hygroscopic

17. Light Collection and Measurement: Light Pipe and PM Tube The exit surface of a scintillator is optically coupled to the PM-tube photocathode through a light pipe, usually consisting of a solid cylinder of polished Lucite The interfaces are filled with an optical coupling agent such as high-viscosity silicone oil or transparent epoxy cement Ideally all materials along the optical path should have nearly the same refractive index as the glass face of the PM tube, ? 1.5

18. Light Pipe and PM Tube (cont.) The main purpose of the light pipe in dosimetry is to remove the PM tube from the radiation field that the scintillator is measuring PM tubes are capable of responding to ionizing events occurring within their structure The interactions occur in different media than the scintillator, at different locations, and with variable gain factors Large doses can so damage a PM tube that its light sensitivity is permanently decreased

19. Comparison with an Ionization Chamber Scintillators are often used as a more sensitive substitute for an ionization chamber in a ?-ray survey meter for health-physics applications It is instructive to consider what factors are involved in estimating the difference in current output from a scintillator and an ion chamber of the same volume

20. Comparison with an Ionization Chamber (cont.) The analogue in a scintillator of ¯W in a gas is the average energy spent by an electron per light photon produced For plastic scintillators this is around 60 eV (about twice that for gases) For good optical coupling ~1/3 of the photons reach the photocathode (typical efficiency about 15% and tube gain about 106) Thus for equal masses of chamber gas and plastic scintillator, the output current of the latter is ~3 ? 104 greater

21. Pulse-Shape Discrimination In most scintillators, the promptly emitted light comprises nearly all of the observed scintillation In some materials a sizable longer-time-constant component exists that is LET-dependent Particles with denser tracks thus have a more pronounced component of longer decay time constant, as shown in the following diagram

23. Pulse-Shape Discrimination (cont.) Suitable electronic discrimination can be provided to count pulses of different lengths separately, correlated with the LET of the particles that produced them Thus it becomes possible to apply different dose calibrations to pulse heights for radiations having different LETs This feature is especially useful for dosimetry in combined neutron-?-ray fields

24. Pulse-Shape Discrimination (cont.) Combinations of two different scintillators coupled to the same PM tube are useful for some dosimetry situations The scintillators chosen have different decay times so pulse-shape discrimination can be applied to separate the signals One thin scintillator can be used to stop a relatively non-penetrating component of radiation while a thicker scintillator behind the first interacts more strongly with more penetrating ?-rays

25. Beta-Ray Dosimetry A plastic scintillator covered by a thin opaque window and coupled to the PM tube face can be used to measure the planar energy-flux density due to incident ?-rays, assuming that the scintillator is thick enough to stop them, and that the light output is proportional to ?-ray energy The distribution of dose vs. depth can be obtained from the reductions in light output observed when a series of tissue-equivalent plastic absorbing layers are placed over the front of the scintillator

26. Semiconductor Detectors: Introduction Semiconductor detectors have characteristics that make them very attractive as dosimeters, for measuring either dose or dose rate, as a substitute for an ion chamber They can also serve as a solid-state analogue of a proportional counter, since the ionization produced by a charged particle in traversing the sensitive volume of the detector is proportional to the energy spent, irrespective of LET, for particles lighter than ?’s

27. Introduction (cont.) Some internal amplification is even possible in the “avalanche detector” mode of operation, but external amplification is usually preferred The broad lack of LET-dependence is an advantage over scintillation detectors, allowing simpler interpretation of pulse heights in terms of energy imparted Semiconductor detectors may be employed as neutron dosimeters by measuring the resulting radiation damage done by the neutrons

28. Basic Operation of Reverse-Biased Semiconductor Junction Detectors The following diagram illustrates the operation of a typical reverse-biased semiconductor, the silicon p-n junction The bulk of the crystal consists of a “p” region having an excess of “holes”, while a thin layer at the surface is an “n” region having an excess of electrons Electrical conduction in each region occurs through motion of these majority charge carriers

30. Reverse-Biased Detectors (cont.) Then a positive potential (~10 – 103 V) is applied to the n-terminal relative to the opposite evaporated-metal surface contact, electrons and holes are pulled out of an intermediate region called the depletion layer, and current cannot then flow across the junction except for some leakage current If a charged particle passes through the depletion layer while the junction is in this reverse-biased condition, it forms electron-hole pairs by the usual collision processes

31. Reverse-Biased Detectors (cont.) The mean energy spent per electron-hole pair in Si at 300 K is 3.62 eV for ?’s and 3.68 for electrons, and in Ge at 77 K it is 2.97 eV for both These figures are only about one-tenth of the analogous W-values for gas ion chambers; hence ~10 times as much ionization is formed in semiconductor detectors as in ion chambers for the same energy expenditure This also helps account for the good energy resolution of Si and Ge detectors

32. Reverse-Biased Detectors (cont.) Electrons have mobilities of 1350 cm/s per V/cm in Si and 3900 in Ge, at 300 K Hole mobilities are 480 cm/s per V/cm in Si and 1900 in Ge, at 300 K Thus typically they can reach the boundary of the depletion layer in 10-7 – 10-8 s, producing a comparable voltage-pulse rise time A charge-sensitive linear preamplifier and linear voltage amplifier comparable to those used for proportional counters, but with suitably shorter time constants, are used to amplify the charge pulses for charge measurement or pulse-height analysis and counting

33. Silicon Diodes without Bias Although the sensitivity is greater and the response time is less for Si diode detectors with reverse bias applied, for DC operation there is an advantage in operating without any external bias: As the bias voltage is reduced to zero, the DC leakage current decreases more rapidly than the radiation-induced current The residual zero-bias radiation-induced current results from alteration of charge-carrier concentrations, and in turn gives rise to a potential difference between the electrodes

36. Silicon Diodes without Bias (cont.) The ranges of dose rate that are measured in radiotherapy applications (0.03 – 3 Gy/min) produce adequate output currents from an unbiased silicon diode detector with a typical sensitivity of ~2 ? 10-11 A per R/min

37. Lithium-Drifted Si and Ge Detectors These are prepared by diffusing Li+ ions into high-purity (but slightly p-type) Si or Ge crystals The Li+ ions lodge at interstitial positions next to the electron-acceptor sites, then capture electrons to become electron donor sites, which thereby neutralize the acceptor sites The crystal is then said to be compensated, by having the same number of electrons in the conduction band as it has holes in the valence band

38. Lithium-Drifted Detectors (cont.) In this condition it acts like an intrinsic material, that is, one that is free of all donor and acceptor sites, being almost completely pure Drifted regions up to almost 2 cm in thickness can be achieved in this way, and the entire intrinsic volume acts as the dosimeter’s sensitive volume Changing the applied potential varies the electric field strength across this volume, but doesn’t change its depth

39. Lithium-Drifted Detectors (cont.) Si(Li) and Ge(Li) detectors can be made as thin as 10 ?m to serve as “dE/dx” measuring devices for charged particles passing through, by which is meant that they respond proportionally to the collision stopping power of the material (ignoring ?-ray production) Likewise they can serve as thin dosimeters, or to measure LET distributions of charged-particle fields

40. Lithium-Drifted Detectors (cont.) Ge(Li) detectors are preferred over Si(Li) for x- or ?-ray spectrometry above 50 keV, or for energy-fluence measurements, because the higher Z (32) of Ge gives it a greater photoelectric cross section than Si (Z = 14), so that Ge stops the beam more efficiently Si(Li) detectors are preferred for lower-energy x rays and for ?-ray dosimetry because their backscattering is much less

41. Lithium-Drifted Detectors (cont.) One disadvantage of Ge(Li) and Si(Li) detectors is that, to maintain their energy resolution for spectrometry, they must be maintained and operated at liquid-N2 temperature Allowing Ge(Li) detectors ever to warm up to room temperature deteriorates them by allowing the Li ions to migrate, thus disturbing donor-acceptor compensation Si(Li) detectors usually may be allowed to reach room temperature without damage, because of lower Li-ion mobility

42. Use of Si(Li) as an Ion-Chamber Substitute The density of Si is about 2.3 g/cm3, or about 1800 times that of air Thus, considering also the ¯W difference, a Si(Li) detector will produce about 18,000 times as much charge as an ion chamber of the same volume, in the same x-ray field, at energies (> 100 keV) where the photoelectric effect is unimportant

43. Use of Si(Li) Junctions with Reverse Bias as Counting Dose-Rate Meters Si(Li) detectors 1 mm thick have been used as probes for measuring the depth dose due to heavy charged particles, including pions The pulse height was found to be proportional to the energy spent by the particle in the sensitive volume of the detector Dose vs. LET results have been consistent with those of a Rossi proportional counter

44. Fast-Neutron Dosimetry Silicon detectors are damaged by very high doses (> 104 Gy) of electrons or x rays, but are much more sensitive to damage by fast neutrons Doses of 0.1 to 10 Gy (tissue) cause permanent defects in the Si crystal lattice, which act as traps for charge carriers As a result the resistance of the detector is effectively increased The voltage drop across the detector when a constant test current is passed through it in a forward direction increases gradually vs. dose