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Outline • Neutron Interactions and – Thermal Dosimetry – Intermediate-energy neutrons – Fast neutrons Chapter 16 • Sources of neutrons • Mixed field dosimetry, paired dosimeters F.A. Attix, Introduction to Radiological • Rem meters Physics and Dosimetry

Introduction Tissue composition • Consider neutron interactions with the majority tissue elements H, O, C, and N, and the resulting • Because of the short ranges of the secondary charged particles that are produced in such interactions, CPE is usually well approximated • Since no bremsstrahlung x-rays are generated, the • The ICRU composition for muscle has been assumed in absorbed dose can be assumed to be equal to the most cases for neutron-dose calculations, lumping the kerma at any point in neutron fields at least up to 1.1% of “other” minor elements together with oxygen to an energy E ~ 20 MeV make a simple four-element (H, O, C, N) composition

Neutron kinetic energy Neutron kinetic energy

• Neutron fields are divided into three • Thermal neutrons, by definition, have the most probable categories based on their kinetic energy: kinetic energy E=kT=0.025eV at T=20C – Thermal (E<0.5 eV) • Neutrons up to 0.5eV are considered “thermal” due to simplicity of experimental test after they emerge from – Intermediate-energy (0.5 eV10 keV) • ratio test: • Differ by their primary interactions in tissue – foil can be activated through 197Au(n,)198Au interaction and resulting biological effects – Addition of 1mm thick Cd filter, which absorbs all neutrons below 0.5eV, tests for presence of those neutrons

1 Neutron kinetic energy Kerma calculations

• For E<10 keV the dose is mainly due to - • For a mono-energetic neutron beam having rays resulting from thermal- fluence  (cm-2), the kerma that results from a in , 1H(n,)2H neutron at a point in a medium is 8 1 • For E>10 keV the dose is dominated by the K 1.60210 Nt m Etr contribution of recoil resulting from where  is the interaction cross section in of hydrogen nuclei 2 cm /(target ), Nt is the number of target – Resulting biological effect (and the in the irradiated sample, m is the sample in corresponding neutron quality factor) rises grams, and Etr is the total kinetic energy (MeV) steeply given to charged particles per interaction

Kerma calculations Kerma calculations

-8 -1 • The product of (1.602  10 Ntm Etr) is • Fn values are tabulated in Appendix F 2 equal to the kerma factor Fn in rad cm /n • Fn is not generally a smooth function of Z and E, • Thus the equation reduces to unlike the case of interaction coefficients • Interpolation vs. Z cannot be employed to obtain CPE values of Fn for elements for which data are not D  K  Fn listed, and interpolation vs. E is feasible only • CPE condition is usually well-approximated within energy regions where resonance peaks are for neutron interactions in tissue absent

Typical neutron absorption Kerma calculations cross section

Resonance peaks occur when the binding energy of a neutron plus the KE of the neutron are exactly equal to the amount required to raise a compound nucleus from its ground state to one of quantum levels

Image from Kerma per unit fluence due to various interaction in a small mass of tissue http://nuclearpowertraining.tpub.com/h1019v1/css/h1019v1_113.htm

2 Thermal-neutron interactions Thermal-neutron interactions in tissue in tissue • Two important interactions of thermal • The nitrogen interaction releases a kinetic energy of neutrons with tissue: Etr = 0.62 MeV that is shared by the (0.58 MeV) and the recoiling nucleus (0.04 MeV) – neutron capture by nitrogen, 14N(n,p)14C • CPE exits since the range of protons in tissue ~10mm – neutron capture by hydrogen, 1H(n,)2H • Based on known values for  and N /m, kerma factor • Thermal neutrons have a larger probability t F  2.741011 [rad cm 2 / n] of capture by hydrogen atoms in muscle, n because there are 41 times more H atoms • Dose deposited CPE than N atoms in tissue D  2.741011 [rad]

Thermal-neutron interactions Thermal-neutron interactions in tissue in tissue • In the hydrogen interaction the energy given to - • In the center of a 1-cm diameter sphere of tissue the rays per unit mass and per unit fluence of thermal kerma contributions from the (n, p) and (n, ) neutrons can be obtained similarly, but replacing processes are comparable in size • In a large tissue mass (radius > 5 times the -ray Etr by E = 2.2 MeV (the -ray energy released in each neutron capture) MFP) where radiation equilibrium is approximated, the kerma due indirectly to the (n, ) process is 26 R m  7.131010 [rad cm 2 / n] • The energy available:  times that of the nitrogen (n, p) interaction • These -rays must interact and transfer their • The human body is intermediate in size, but large energy to charged particles to produce kerma enough so the 1H(n, )2H process dominates in • Contribution to dose depends on proximity to RE kerma (and dose) production

Interactions by intermediate Interactions by neutrons in tissue and fast neutrons in tissue • It is dominated below 10-4 MeV (100eV) by (n,p) reactions, mostly in nitrogen, represented by curve g • Above 10-4 MeV elastic scattering of hydrogen nuclei (curve b) contributes nearly all of the kerma x10-3

3 Interaction by intermediate Interaction by intermediate and fast neutrons in tissue and fast neutrons in tissue • The average energy transferred by elastic • Values for E for different tissue atoms: scattering to a nucleus is closely approximated tr (i.e., assuming isotropic scattering in the center- – E/2 for hydrogen recoils (changes from 0 when o of-mass system) by proton is recoiling at 90 to Etr for protons recoiling straight ahead) 2M M a n – 0.142E for C atoms Etr  E 2 M a  M n  – 0.124E for N where E = neutron energy, – 0.083E for O

Ma = mass of target nucleus, Mn = neutron mass

Neutron sources Neutron sources • Neutrons can be released through processes such as fission, , or neutron stripping • Most widely available neutron sources: • Fission: splitting atoms in a reactors – n + 235U = n + n + fragments – one n may go back – Accelerators into chain reaction, the other is available – Radioactive sources • Spallation: bombarding heavy atoms with energetic protons – p + heavy nucleus = X * n + fragments • Stripping: bombarding metal atoms with protons, a-particles, etc.

Neutron sources Neutron sources • Fission-like spectra have average neutron energies ~2MeV • From beam ports of nuclear reactors • Watt spectrum is an idealized shape for unmoderated reactor neutrons • Several types of Be(a,n) radioactive sources are in common use, employing 210Po, 239Pu, 241Am, or 226Ra as the emitter • Neutron yields are on the order of 1 neutron per 104 a- Fission neutron spectra from reactors particles; average energies  4 MeV

4 Neutron sources Neutron Quality Factor deuteron bombardment of Be target

• Cyclotrons are used to produce neutron beams by accelerating protons or deuterons into various targets, most commonly Be • For purposes of protection the dose equivalent H = DQ, • The neutron spectrum extends from 0 to energies Emax somewhat above the where Q is the quality factor, which depends on neutron energy deuteron energy, and have E ~0.4E avg max • The quality factor for all -rays is taken to be unity for purposes of • Tissue dose rates ~10rad/min at 1m in tissue, -ray background ~few % combining neutron and -ray dose equivalents

Radiation Weighting Factor Mixed-Field Dosimetry n + 

• New approach uses radiation and tissue weighting factors • Neutrons and -rays are both indirectly ionizing (updated in ICRP 103 report, 2007); effective dose that are attenuated exponentially in E=w H = w w *D T T T R T,R passing through matter • For neutrons weighting factor is a continuous function of energy; represented as a graph or in functional form • Each is capable of generating secondary fields of the other radiation: – (, n) reactions are only significant for high-energy - rays (10 MeV, there is a threshold defined by reaction) – (n, ) reactions can proceed at all neutron energies and are especially important in thermal-neutron capture, as discussed for 1H(n, )2H

Mixed-Field Dosimetry n +  Mixed-Field Dosimetry n + 

• Neutron fields are normally “contaminated” by • Three general categories of dosimeters for secondary -rays n +  applications: • Since neutrons generally have more biological 1. Neutron dosimeters that are relatively effectiveness per unit of absorbed dose than - insensitive to  rays rays, it is desirable to account for  and n 2. -ray dosimeters that are relatively insensitive components separately to neutrons • It is especially important in the case of neutron 3. Dosimeters that are comparably sensitive to dosimeters to specify the reference material to both radiations which the dose reading is supposed to refer

5 Mixed-Field Dosimetry n +  Mixed-Field Dosimetry n + 

• While is a very close substitute of muscle • The response of a dosimeter to a mixed field of neutrons and -rays tissue in photon dosimetry, it is not as close a Q  AD  BD substitute for neutrons n,  n • Water is 1/9 hydrogen by weight; muscle is 1/10 or alternatively as hydrogen Q B n,  D  D • Water contains no nitrogen, and hence can have no A  A n 14N(n,p)14C reactions by thermal neutrons Here A = response per unit of absorbed dose in tissue for - rays, B = response per unit of absorbed dose in tissue for • For 1MeV photon (men/r)muscle=0.99 (men/r)water , for 1MeV neutron (F ) =0.91(F ) neutrons, D and Dn are absorbed doses in tissue n muscle n water correspondingly due to -rays and neutrons

Mixed-Field Dosimetry n +  Mixed-Field Dosimetry n + 

• By convention the absorbed dose referred to in • An approach of employing two dosimeters (paired these terms is assumed to be that under CPE dosimeters) having different values of A/B can be

conditions in a small imaginary sphere of muscle used to simultaneously obtain D and Dn tissue, centered at the dosimeter midpoint with the • The best dosimeter pair is a TE-plastic ion chamber dosimeter absent containing TE gas (for which B/A  1) to measure • Most commonly this tissue sphere is taken to be the total n +  dose, and a nonhydrogenous just large enough (0.52-g/cm2 radius) to produce dosimeter having as little neutron sensitivity as possible to measure the  dose CPE at its center in a 60Co beam – Ideally this paired dosimeter should measure only -rays • Closer values of A/B in a pair decrease the accuracy

Mixed-Field Dosimetry n +  Activation of metal foils

• Often dosimeters insensitive to -rays are used in • Since most of radio-activation by photonuclear mixed fields to evaluate neutron dose only (A<10MeV), metal foils are only activated by – Fission foils (A = 0) the neutrons in the mixed field – Etchable plastic foils (A  0) • The resulting activity of a foil is measured by – Damage to silicon diodes (A  0) counting -rays emitted (G-M counter will work) – Hurst proportional counter (A  0) • Some activated foils are b-emitters (e.g., in – Rem meters 32S(n,p)32P reaction, 32P is a b-emitter) – Long counters – Bubble detectors

6 Activation of metal foils Rem meters

• Thermal neutrons have a fixed activation cross section (E)= • Fast neutrons have activation threshold, below which (E)=0 • Since different materials have different threshold, a set of foils with different thresholds allows determination of the neutron spectrum

Rem meters Rem meters

The maximum value of the dose equivalent, [3600d(E)]-1 in the body (tissue slab in 30-cm thickness) vs energy of neutrons

Rem meters Summary • Neutron dosimetry approaches differ from those of photon dosimetry • Water is not the best tissue-mimicking phantom anymore

Sectional view of • Quality factor for neutrons is energy Andersson-Braun rem dependent counter • Dosimetry involves measurement of mixed n+ fields

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