Outline. Neutron Interactions and Dosimetry. Introduction. Tissue composition. Neutron kinetic energy. Neutron kinetic energy.

Transcription

1 Outlie Neutro Iteractios ad Dosimetry Chapter 16 F.A. Attix, Itroductio to Radiological Physics ad Radiatio Dosimetry Neutro dosimetry Thermal eutros Itermediate-eergy eutros Fast eutros Sources of eutros Mixed field dosimetry, paired dosimeters Itroductio Cosider eutro iteractios with the majority tissue elemets H, O, C, ad N, ad the resultig absorbed dose Because of the short rages of the secodary charged particles that are produced i such iteractios, CPE is usually well approximated Sice o bremsstrahlug x-rays are geerated, the absorbed dose ca be assumed to be equal to the kerma at ay poit i eutro fields at least up to a eergy E ~ 20 MeV Tissue compositio The ICRU compositio for muscle has bee assumed i most cases for eutro-dose calculatios, lumpig the 1.1% of other mior elemets together with oxyge to make a simple four-elemet (H, O, C, N) compositio Neutro kietic eergy Neutro fields are divided ito three categories based o their kietic eergy: Thermal (E<0.5 ev) Itermediate-eergy (0.5 ev<e<10 kev) Fast (E>10 kev) Differ by their primary iteractios i tissue ad resultig biological effects Neutro kietic eergy Thermal eutros, by defiitio, have the most probable kietic eergy E=kT=0.025eV at T=20C Neutros up to 0.5eV are cosidered thermal due to simplicity of experimetal test after they emerge from moderator material Cadmium ratio test: Gold foil ca be activated through 197 Au(, ) 198 Au iteractio Additio of 1mm thick Cd filter, which absorbs all eutros below 0.5eV, tests for presece of those eutros 1

2 Neutro kietic eergy For E<10 kev the dose is maily due to - rays resultig from thermal-eutro capture i hydroge, 1 H(, ) 2 H For E>10 kev the dose is domiated by the cotributio of recoil protos resultig from elastic scatterig of hydroge uclei Resultig biological effect (ad the correspodig eutro quality factor) rises steeply Kerma calculatios For a moo-eergetic eutro beam havig fluece (cm -2 ), the kerma that results from a eutro at a poit i a medium is K N m 1 t E tr where is the iteractio cross sectio i cm 2 /(target atom), N t is the umber of target atoms i the irradiated sample, m is the sample mass i grams, ad E tr is the total kietic eergy (MeV) give to charged particles per iteractio Kerma calculatios The product of ( N t m -1 E tr ) is equal to the kerma factor F i rad cm 2 / Thus the equatio reduces to CPE D K F CPE coditio is usually well-approximated for eutro iteractios i tissue Kerma calculatios F values are tabulated i Appedix F F is ot geerally a smooth fuctio of Z ad E, ulike the case of photo iteractio coefficiets Iterpolatio vs. Z caot be employed to obtai values of F for elemets for which data are ot listed, ad iterpolatio vs. E is feasible oly withi eergy regios where resoace peaks are abset Typical eutro absorptio cross sectio Kerma calculatios Resoace peaks occur whe the bidig eergy of a eutro plus the KE of the eutro are exactly equal to the amout required to raise a compoud ucleus from its groud state to oe of quatum levels Image from Kerma per uit fluece due to various iteractio i a small mass of tissue 2

3 Thermal-eutro iteractios i tissue Two importat iteractios of thermal eutros with tissue: eutro capture by itroge, 14 N(,p) 14 C eutro capture by hydroge, 1 H(, ) 2 H Thermal eutros have a larger probability of capture by hydroge atoms i muscle, because there are 41 times more H atoms tha N atoms i tissue Thermal-eutro iteractios i tissue The itroge iteractio releases a kietic eergy of E tr = 0.62 MeV that is shared by the proto (0.58 MeV) ad the recoilig ucleus (0.04 MeV) CPE exits sice the rage of protos i tissue ~10mm Based o kow values for ad N t /m, kerma factor [rad cm / ] F Dose deposited CPE D [rad] Thermal-eutro iteractios i tissue I the hydroge iteractio the eergy give to - rays per uit mass ad per uit fluece of thermal eutros ca be obtaied similarly, but replacig E tr by E = 2.2 MeV (the -ray eergy released i each eutro capture) 10 2 The eergy available: R m [rad cm / ] These -rays must iteract ad trasfer their eergy to charged particles to produce kerma Cotributio to dose depeds o proximity to RE Thermal-eutro iteractios i tissue I the ceter of a 1-cm diameter sphere of tissue the kerma cotributios from the (, p) ad (, ) processes are comparable i size I a large tissue mass (radius > 5 times the -ray MFP) where radiatio equilibrium is approximated, the kerma due idirectly to the (, ) process is 26 times that of the itroge (, p) iteractio The huma body is itermediate i size, but large eough so the 1 H(, ) 2 H process domiates i kerma (ad dose) productio Iteractios by itermediate ad fast eutros i tissue It is domiated below 10-4 MeV (100eV) by (,p) reactios, mostly i itroge, represeted by curve g Above 10-4 MeV elastic scatterig of hydroge uclei (curve b) cotributes early all of the kerma Iteractios by eutros i tissue x10-3 3

4 Iteractio by itermediate ad fast eutros i tissue The average eergy trasferred by elastic scatterig to a ucleus is closely approximated (i.e., assumig isotropic scatterig i the ceterof-mass system) by E tr E 2M M M M 2 where E = eutro eergy, M a = mass of target ucleus, M = eutro mass a a Iteractio by itermediate ad fast eutros i tissue Values for for differet tissue atoms: E/2 for hydroge recoils (chages from 0 whe proto is recoilig at 90 o to E tr for protos recoilig straight ahead) 0.142E for C atoms 0.124E for N 0.083E for O E tr Neutro sources Neutros ca be released through processes such as fissio, spallatio, or eutro strippig Fissio: splittig atoms i a uclear reactor U = + + fragmets oe may go back ito chai reactio, the other is available Spallatio: bombardig heavy metal atoms with eergetic protos p + heavy ucleus = X * + fragmets Strippig: bombardig light metal atoms with protos, a-particles, etc. Neutro sources Most widely available eutro sources: Nuclear fissio reactors Accelerators Radioactive sources Neutro sources Fissio eutro spectra from reactors Fissio-like spectra have average eutro eergies ~2MeV From beam ports of uclear reactors Watt spectrum is a idealized shape for umoderated reactor eutros Neutro sources Several types of Be(a,) radioactive sources are i commo use, employig 210 Po, 239 Pu, 241 Am, or 226 Ra as the emitter Neutro yields are o the order of 1 eutro per 10 4 a- particles; average eergies 4 MeV 4

5 Neutro sources deutero bombardmet of Be target Neutro Quality Factor Cyclotros are used to produce eutro beams by acceleratig protos or deuteros ito various targets, most commoly Be The eutro spectrum exteds from 0 to eergies E max somewhat above the deutero eergy, ad have E avg ~0.4E max Tissue dose rates ~10rad/mi at 1m i tissue, -ray backgroud ~few % For purposes of eutro radiatio protectio the dose equivalet H = DQ, where Q is the quality factor, which depeds o eutro eergy The quality factor for all -rays is take to be uity for purposes of combiig eutro ad -ray dose equivalets Radiatio Weightig Factor New approach uses radiatio ad tissue weightig factors (updated i ICRP 103 report, 2007); effective dose E=w T H T = w T w R *D T,R For eutros weightig factor is a cotiuous fuctio of eergy; represeted as a graph or i fuctioal form Neutros ad -rays are both idirectly ioizig radiatios that are atteuated expoetially i passig through matter Each is capable of geeratig secodary fields of the other radiatio: (, ) reactios are oly sigificat for high-eergy - rays ( 10 MeV, there is a threshold defied by reactio) (, ) reactios ca proceed at all eutro eergies ad are especially importat i thermal-eutro capture, as discussed for 1 H(, ) 2 H Neutro fields are ormally cotamiated by secodary -rays Sice eutros geerally have more biological effectiveess per uit of absorbed dose tha - rays, it is desirable to accout for ad compoets separately It is especially importat i the case of eutro dosimeters to specify the referece material to which the dose readig is supposed to refer Three geeral categories of dosimeters for + applicatios: 1. Neutro dosimeters that are relatively isesitive to rays 2. -ray dosimeters that are relatively isesitive to eutros 3. Dosimeters that are comparably sesitive to both radiatios 5

6 While water is a very close substitute of muscle tissue i photo dosimetry, it is ot as close a substitute for eutros Water is 1/9 hydroge by weight; muscle is 1/10 hydroge Water cotais o itroge, ad hece ca have o 14 N(,p) 14 C reactios by thermal eutros For 1MeV photo (m e /r) muscle =0.99 (m e /r) water, for 1MeV eutro (F ) muscle =0.91(F ) water The respose of a dosimeter to a mixed field of eutros ad -rays Q, AD BD or alteratively as Q, B D D A A Here A = respose per uit of absorbed dose i tissue for - rays, B = respose per uit of absorbed dose i tissue for eutros, D ad D are absorbed doses i tissue correspodigly due to -rays ad eutros By covetio the absorbed dose referred to i these terms is assumed to be that uder CPE coditios i a small imagiary sphere of muscle tissue, cetered at the dosimeter midpoit with the dosimeter abset Most commoly this tissue sphere is take to be just large eough (0.52-g/cm 2 radius) to produce CPE at its ceter i a 60 Co beam A approach of employig two dosimeters (paired dosimeters) havig differet values of A/B ca be used to simultaeously obtai D ad D The best dosimeter pair is a TE-plastic io chamber cotaiig TE gas (for which B/A 1) to measure the total + dose, ad a ohydrogeous dosimeter havig as little eutro sesitivity as possible to measure the dose Ideally this paired dosimeter should measure oly -rays Closer values of A/B i a pair decrease the accuracy Ofte dosimeters isesitive to -rays are used i mixed fields to evaluate eutro dose oly (A<<B) Activatio of metal foils (A 0) Fissio foils (A = 0) Etchable plastic foils (A 0) Damage to silico diodes (A 0) Hurst proportioal couter (A 0) Log couters Bubble detectors Activatio of metal foils Sice most of radio-activatio by photouclear reactios ca occur oly above the eergy rage of - radiatio (>10MeV), metal foils are oly activated by the eutros i the mixed field The resultig activity of a foil is measured by coutig -rays emitted (G-M couter will work) Some activated foils are b-emitters (e.g., i 32 S(,p) 32 P reactio, 32 P is a b-emitter) 6

7 Activatio of metal foils Thermal eutros have a fixed activatio cross sectio (E)= Fast eutros have activatio threshold, below which (E)=0 Sice differet materials have differet threshold, a set of foils with differet thresholds allows determiatio of the eutro spectrum The maximum value of the dose equivalet, [3600d(E)] -1 i the body (tissue slab i 30-cm thickess) vs eergy of eutros Sectioal view of Adersso-Brau rem couter Summary Neutro dosimetry approaches differ from those of photo dosimetry Water is ot the best tissue-mimickig phatom aymore Quality factor for eutros is eergy depedet Dosimetry ivolves measuremet of mixed + fields 7