Neutrons. ρ σ. where. Neutrons act like photons in the sense that they are attenuated as. Unlike photons, neutrons interact via the strong interaction

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1 Neutrons Neutrons act like photons in the sense that they are attenuated as I = I 0 e μx where Unlike photons, neutrons interact via the strong interaction μ = The cross sections are much smaller than those of the electromagnetic interaction because of the short range of the nuclear (strong) force The cross sections are O( barns) N Av A ρ σ tot 1

2 Neutrons Neutrons can be classified by their kinetic energy High energy (> 10 MeV) Fast (500 kev 10 MeV) Intermediate (1 kev 500 kev) Resonance (1 ev 1 kev) Epithermal (0.025 ev 1 ev) Thermal (slow) ~ ev = kt Cold (< kt) 2

3 Resonance Neutrons 238 U cross sections 3

4 Neutrons There are a variety of neutron interactions with nuclei from low to high energy these are (n,γ) neutron capture (n, n) elastic scattering (n,n ) inelastic scattering (n,p), (n,α) charged particle emission (n,f) fission (n,x) hadronic shower production 4

5 Neutrons High energy neutrons produce hadronic showers Important for calorimetry in particle physics Fast neutrons interact principally by elastic scattering Resonance neutrons can produce resonances (long-lived excited states) Thermal neutrons are captured Cross section ~ 1/v (1/v law is good to ~ 1000 ev) 5

6 Neutrons Cross section falls as 1/v at low energies 6

7 Neutrons Neutrons are usually detected by the charged particles they produce Recoil nucleus Proton Alpha particle Fission fragments Note these particles are heavy and thus produce significant ionization Neutrons (high LET radiation) Photons (low LET radiation) 7

8 Neutron Elastic Scattering Elastic scattering is the most important process for slowing down neutrons Using KE and momentum conservation one can show that the maximum energy that a neutron of mass m and kinetic energy E 0 can transfer to a nucleus of mass M in an elastic collision is 4mM Q max = ( M + m) 2 E 0 8

9 Neutron Elastic Scattering Some examples of the maximum fractional energy loss by a neutron How would you design a shield for neutrons? Nucleus 1 H 2 H 4 He 16 O 56 Fe 238 U Q max /E

10 Neutron Inelastic Scattering Inelastic total cross section for iron 10

11 Neutrons Thermal neutron detectors are based on the detection of slow alphas or protons through 10 B+ 1 n 7 Li * + 4 α 6 Li+ 1 n 3 H + 4 α 3 He+ 1 n 3 H + 1 p 235 U, 239 Pu fission 11

12 Neutrons 12

13 Neutron Detectors BF 3 and 3 He proportional counters Large cross sections (3840 and 5330 barns for thermal neutrons) Q-value released determines kinetic energy of daughters through energy and momentum conservation Gas detectors show the wall effect if they are of limited size No information about the energy spectrum of the incident neutrons is possible 13

14 3 He Proportional Counter Principle of operation 14

15 3 He Proportional Counter Wall effect gives a characteristic energy spectrum that can be used for gamma ray discrimination 15

16 Neutron Detectors Neutron inorganic scintillators All daughter products are absorbed But so are photons so poor discrimination 16

17 Neutron Detectors Fast neutrons can be detected by Moderation to thermal energies Bonner spheres Recoil proton in hydrogen (more common) Organic scintillators Can use proton TOF to determine 17

18 Bonner Spheres 6 LiI scintillator 18

19 Bonner Spheres 19

20 Neutron Detectors CVD (Chemical Vapor Deposition) diamond detectors For extreme radiation environments Fusion installations (ITER and NIF) HEP experiments (slhc) Proton and carbon-beam therapy monitoring Both polycrystalline and single crystal CVD is possible 20

21 CVD Diamond mm pcvd diamond films using CO 2 /CH 4 plasmas 21

22 CVD Diamond Principle of operation 22

23 Pros CVD Diamond Intrinsically radiation hard Low dielectric constant so very fast (ns) charge collection Atomic number close to tissue Low Z means less sensitivity to gamma rays Cons High energy needed to produce electron-hole pair 13.2 ev for diamond versus 3.6 ev for silicon Small signal requires preamplification close to the detector Several types of electron/hole traps in the diamond film Response reproducibility of films 23

24 Neutron Therapy Can neutrons be used for radiation therapy? Depth dose for a 66 MeV proton beam incident on a Be target (equivalent to 8 MV photon beam) 24

25 Positives Neutron Therapy High-LET radiation High RBE radiation (1.5-5)? Relative Biological Effect == ratio of doses required to achieve the same biological effect Smaller doses, shorter treatment time More effective on radioresistant tumors Low oxygen tension tumors Repair of radiation damage Negatives Unclear if systematic studies support these positives 25

26 Neutron Therapy RBE 26

27 Neutron Therapy Soft tissue sarcoma 27

28 Boron Neutron Capture Therapy Proposed by Locher in 1936, only a few years after the discovery of the neutron Idea is to treat (kill) individual tumor cells, mainly gliomas (brain tumor) Tag tumor cells with boron ( 10 B) (~20 μg / g) Irradiate with epithermal neutrons (0.5 ev 1 kev) Produce high LET and high RBE products which kill the individual cell * 4 7 * B + n B He+ Li MeV 28

29 Boron Neutron Capture Therapy The BNCT reaction The heavy daughters travel 5-9 μm which is O(cell diameter) 29

30 Boron Neutron Capture Therapy Radiation dose delivered to tumor and normal tissues are High LET decay products of 11 B and 7 Li High LET protons from 14 N(n,p) 14 C Low LET photons from 1 H(n,γ) 2 H 30

31 Boron Neutron Capture Therapy Examples of boron delivery agents 31

32 Boron Neutron Therapy Capture Concentration of BPA 32

33 Boron Neutron Therapy Capture Still in trials mainly for gioblastoma multiforme 33

34 Boron Neutron Capture Therapy Still in trials mainly for gioblastoma multiforme 34

35 35 Neutrons Fast neutrons can be problematic in shielding or calorimetry because they only elastic scatter How best to shield a fast neutron? polyethylene or lead? Using momentum and energy conservation for m 1 and m 2 elastic scattering 1 finds 0 large finds = + = = f m m f m m m m m T T T f i f i

36 Neutrons Activation energy for fission 36

37 Neutrons Inelastic total cross section for iron 37