Radiation Detection and Measurement

1. Radiation Detection and Measurement 核辐射探测 刘义保 王玲 东华理工大学 核工程技术学院

2. Chapter 1 Radiation Sources • 1.1 Units and Definitions • 1.2 Fast Electron Sources • 1.3 Heavy Charged Particles Sources • 1.4 Sources of Electromagnetics Radiation • 1.5 Neutron Sources • A • The radiations are conveniently categorized into four general types as follows: • Charged particulate radiation Fast electrons • Heavy charged particles • Uncharged radiation Electromagnetic radiation • Neutrons

3. Some concerns: Fast electron: include beta particles (positive or negative) emitted in nuclear decay, as well as energetic electrons produced by any other process. Heavy charged particles: denoted a category that encompasses all energetic ions with mass of one atomic mass unit or greater, such as alpa particle, proton, fission products, or the products of many nuclear reactions. Electromagnetic radiations: include x-rays emitted in the rearrangement of electron shell of atoms, and γ-rays which originated from transitions within the nucleus itself.

4. Neutron: generated in various nuclear processes comprise the final major category, which is often divided into slow neutron and fast neutron subcategories. Ionizing radiation: The energy range of our interest spans over six decades, ranging from about 10eV to 20MeV. The lower energy bound is set by the minimum energy required to produce ionization in typical materials by the radiation or the secondary products of its interaction. Radiation with greater than this minimum are classified as ionizing radiation.

5. B The radiations of interest differ in their “hardness” or ability to penetrate thicknesses or material. soft radiation: such as alpha, x-ray (lower energy), penetrate only small thickness of material. harder radiation: such as gamma, neutrons. between soft and harder radiation: such as beta.

6. 1 Units and Definitions • Radioactivity • The activity of a radioisotope source is defined as its rate of decay and is given by the fundmental law of radioactive decay. Where N – the number of radioactive nuclei λ – the decay constant The unit of activity: The history unit: Cure (Ci) , 1Ci=3.7x1010 disintegrations/second SI unit: 1Bq=2.703x10-11Ci

7. The specific activity: It is defined as the activity per unit mass of the radioisotope sample. For pure or “carrier-free” sample, the specific activity can be calculated from: Where M=Molecular weight of sample Av=Avogadro’s number (=6.02x1023nuclei/mole) λ =radioisotope decay constant (=ln2/half-life)

8. B. Energy The traditional unit for the measurement of radiation energy is the electron volt or eV, (keV, MeV for ionizing radiation). The SI unit of energy is joule(J): 1 fJ(10-15J)=6.241x103eV The energy of an x-ray or γ-ray photon is related to the radiation frequency by: Where h=Planck constant (6.626x10-34Js, or, 4.135x10-15eVs). v=frequency The wavelength λ is related to the photon energy by: Where λ is in meters and E in eV.

9. 2 Fast Electron Sources • Beta Decay • The most common source of fast electron in radiation measurement is a radioisotope that decay by beta-minus emission. Where X, Y are the initial and final nuclear species. is the anti-neutrino Each species beta decay transition is characterized by a fixed decay energy or Q-value (beta endpoint energy Fig.1-1). The beta particle energy varies from decay to decay and can be range from zero to Q-value, see table 1-1.

10. Table 1-1 Some “Pure” Beta-Minus Sources

11. Figure 1-1 The decay scheme of 36Cl and the resulting beta particle energy distribution.

12. B. Internal Conversion The continuum of energies produced by any beta source is inappropriate for some applications. For example, if an energy calibration is to be carried out for an electron detector, it is much more convenient to use a source of monoenergetic electrons. The nuclear process of internal conversion can be the source of conversion electrons, which are, under some circumstances, nearly monoenergetic. The internal conversion is the preceding process of beta decay of a parent species, where, the nuclear excitation energy Eex is transferred directly to one of the orbital electrons of the atom. This electron then appears with an energy given by Where Eb is its binding energy in the original electron shell.

15. C. Auger Electron Auger electrons are roughly the analogue of internal conversion electrons when the excitation energy originates in the atom rather than in the nucleus. A preceding process (such as EC) may leave the atom with a vacancy in a normally complete electron shell. This vacancy is often filled by electrons from the outer shell of the atom with the emission of a characteristic X-ray photon. Alternatively, the excitation energy of the atom may be transferred directly to one of the outer electron, causing it to be ejected from the atom. This electron is called Auger electron and appears with an energy given by the difference between the original atomic excitation energy and the binding energy of the shell from which the electron was ejected. Auger electrons produce a discrete energy spectrum, with different groups corresponding to different initial and final states.

16. In all cases, the Auger electrons energy is relatively lower than that of beta particles or conversion electrons.

17. 1.3 Heavy Charged Particle Sources • Alpha Decay The probability of decay is governed by the barrier penetration mechanism described in Quantum Mechanics, the half-life of useful sources varies from days to many thousands of years.

19. Notes: 1) alpha particles appear one or more energy groups. 2) most alpha particles energies are limited between 4 and 6 MeV. 3) 241Am is mostly used to calibrate the source for alpha particle. 4) the detector is commonly silicon solid-state detector.

20. 241Am 5.389MeV(1.0%) 5.443MeV(12.5%) 5.869MeV(86.0%) 0.156 5.545MeV(0.3%) γ0.097(0.02%) 0.102 γ0.043(0.1%) 0.059 γ0.102(0.02%) γ0.059(36%) 0.000 237Np Fig. 1-3’ alpha particle groups produced in the decay of 241Am

22. B. Spontaneous Fission • The fission process is the only spontaneous source of energetic heavy charged particles with mass greater than that of alpha particle. • The most widely used example is 252Cf. see fig.1-4(a) (b).

25. 1-4. Source of Electromagnetic Radiation A. Gamma Rays Following Beta Decay Gamma radiation is emitted by excited nuclei in their transition to lower lying nuclear levels. Notes: 1) For common examples widely used as gamma ray calibration sources. In each case, a form of beta decay leads to the population of excited state in the daughter nucleus. 2) The gamma rays from any one transition are nearly monoenergetic. 3) The common gamma ray source based in beta decay are generally limited to energies below about 2.8 MeV.

27. B. Annihilation Radiation When the parent nucleus undergoes beta-plus decay, additional electromagnetic radiation is generated. The origin lie in the fate of the positrons emitted in the primary decay process. Annihilation Radiation: The original positron and electron disappear and are replaced by two oppositely directed 0.511MeV electromagnetic photons known as annihilation radiation. Example: the decay of 22Na:

28. C. Gamma Ray Following Nuclear Reactions • If gamma rays with energies higher than those available from beta-active isotopes are needed, some other process must lead to the population of higher-lying nuclear states. • Example 1: ~4.44MeV Example 2: ~6.130MeV

29. D. Bremsstrahlung When fast electrons interaction in matter, part of their energy is converted into electromagnetic radiation in the form of Bremsstrahlung. Notes: 1) For monoenergetic electron that slow down and stop in a given material, the Bremsstrahlung energy spectrum is a continuum with photon energies that extend as high as the electron energy itself. 2) The shape of the energy spectrum from an x-ray tube can be beneficially alter by filtration or passage through appropriate absorber materials. 3) Bremsstrahlung is also produced by other source of fast electrons, including beta particles. I4) In addition to Bremsstrahlung, characteristic x-rays are also produced when fast electron pass through an absober. Therefore, the spectra from x-ray tubes or other Bremsstrahlung sources also show characteristic x-ray emission lines superimposed on the continuous Bremsstrahlung spectrum.

30. E. Characteristic X-Rays If the orbital electrons in an atom are disrupted from their normal configuration by some excitation process, the atom may be exist in an excited state for a short period of time. There is a nature tendency for the electrons to rearrange themselves to return the atom to its lowest energy state or ground state within a time that is characteristically a nanosecond or less in a solid material. The energy librated in the transition from the excited to the ground state takes the form of a characteristic x-ray photon whose energy is given by the energy difference between the initial and final states.

31. Notes: Flourescent yield: For an atom in an excited state, the ejection of Auger electron is a competitive process to the emission of characteristic x-ray. The flourescent yield is defined as the fraction of all cases in which the excited atom emits a characteristic x-ray photon in its deexcitation.

32. Classification: 1. Excitation by Radioactive Decay In the nuclear decay process of electron capture the nuclear charged is decreased by one unit by the capture of an orbital electron (EC). The capture process has created a vacancy in one of the inner shell. when this vacancy is subsequently filled, X-rays are generated which are characteristic of the product electron. Notes: 1) The decay may populate either the ground state or an excited state in the product nucleus, so that the characteristic X-rays may also be accompanied by r-rays from subsequent nuclear deexcitation. 2) Internal conversion is another nuclear process that can lead to characteristic X-rays. 3) The conversion electrons may also lead to a measurable bremsstrahlung continuum, particuarly when their energy is high.

34. 2. Excitation by External Radiation • An external source of radiation (x-rays, electrons, α particles, etc. ) may also be used to generate characteristic x-rays.

35. Notes: ①The energy of x-rays depends on Low atomic number →soft x-rays, Target material High z→harder or higher energy x-rays. ②X-ray fluorescence The incident radiation may consist of x-rays generated in a conventional x-ray tube. These x-rays may then interact in the target through photoelectron, and the subsequent deexcitation of the target ions creates their characteristic x-rays. This process is called x-ray fluorescence.  As an alternative to bulky x-ray tubes, radioisotopes that emit low-energy photons may also be used as the source of the excitation. An example is 241Am, in which gamma rays of 60KeV energy are emitted in 36% of the decay, see fig1-3’. Characteristic x-ray sources using this isotope for excitation of targets in the geometry shown in figure 1-9 are currently available using up to 10m Ci of 241Am activity.

36. 241Am α5.389MeV(1.0%) α5.443MeV(12.5%) α5.869MeV(86.0%) 0.156 α5.545MeV(0.3%) γ0.097(0.02%) 0.102 γ0.043(0.1%) 0.059 γ0.102(0.02%) γ0.059(36%) 0.000 237Np Fig. 1-3’ alpha particle groups produced in the decay of 241Am.

38.  Another method of exciting the target is through the use of an external electron beam or heavy charged particles. See fig1-8. • 1) accelerator for electron to only a few thousand volts; • 2) isotope source fro α-particles with 210Po, 144Cm.

39. 1-5 Neturon Source A. Spontaneous Fission 1) The most common spontaneous fission source is 252Cf; 2) Half-time:2.65y; 3) the dominant decay mechanism is α decay; 4) the energy spectrum of neutrons is plotted in fig. 1-10.

41. B. Radioisotope (α, n) Sources • Several different target materials can lead to (α, n) reactions for the alpha particle energies which are readily available in radioactive decay. • The maximum neutron yield is obtained with Be as the target: It’s Q-value:5.71MeV. • 2) The 239Pu/Be sources is probably the most widely used of the (α, n) isotopic neutron sources, see table 1-6.

43. C. Photoneutron Sources • Some radioisotope gamma-ray emitters can also be used to produce neutrons when combined with an appropriate target material. The resulting photoneutron sources are based on supplying sufficient excitation energy to a target by absorption of gamma-ray photon to allow the emission of free neutron. • Only two target nuclei, 9Be and 2H, are of any practical signification for radioisotope source.

44. The corresponding neutron energy can be calculated from: Where: θ – angle between gamma photon and nertron direction; E– gamma energy (assumed << 931MeV) M – mass of recoil nucleus x c2. m – mass of neutron x c2.

46. Notes: • 1) The main disadvantage of photoneutron sources: • It is needed that very large gamma-ray activities must be used in order to produce neutron sources of attractive intensity. • one gamma ray (105 or 106) → one neutron • 2) Some of the more common gamma-ray emitters are 226Ra, 124Sb, 72Ga, 140La and 24Na.

48. D． Reactions From Accelerated Charged Particle (“neutron generator”) Because alpha particles are the only heavy charged particles with low z conventiently available from radioisotopes, reactions involving incident protons, deuterons, and so on, must rely on artifically accelerated particles. Two of the most common reaction of this type used to produce neutrons are: 3.26MeV (Q-value) D—D 17.6MeV (Q-value) D—T Notes : 1) 1 mA D (accelerated potentiao 100~300keV) → 109n/s (D-D), 1011n/s(D-T). 2) Other neutron generators (higher energy): 9Be(d,n), 7Li(p,n), 3H(p,n)