radiation detection 2

8/3/2019 Radiation Detection 2

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Ionizing RadiationIonizing Radiation

Detection andDetection andMeasurementsMeasurements

Neutrons DetectionNeutrons DetectionNE 311NE 311


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1.1. NEUTRON KINETIC ENERGYNEUTRON KINETIC ENERGY

ItIt is convenientto divide neutron fields into theis convenientto divide neutron fields into the followingfollowing

energyenergy categoriescategories..A. ThermalA. Thermal NeutronsNeutrons

Thermal neutrons have onlytheThermal neutrons have onlythe MaxwellianMaxwelliandistribution ofthermal motion thatdistribution ofthermal motion that is characteristic ofis characteristic of

thethe temperature ofthe medium in whichthey existtemperature ofthe medium in whichthey exist..

TheirmostTheirmostprobable kineticprobable kinetic energy atenergy at 2O2O00 CC isisEE ==0.0250.025 eVeV..

However, all neutrons withHowever, all neutrons with energies belowenergies below 0.50.5 eVeV areareusuallyreferred to as thermal because of a simpleusuallyreferred to as thermal because of a simpleexperimentalexperimental testthat can be applied to a neutron fieldtestthat can be applied to a neutron field

to measure how completely ithasto measure how completely ithas been been thermalizedthermalized

bypassage through a moderator.bypassage through a moderator.


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B. IntermediateB. Intermediate--EnergyEnergy NeutronsNeutrons

Neutrons with energies above the thermal cutoff of 0.5Neutrons with energies above the thermal cutoff of 0.5eVeV but below 10but below 10 keVkeV areare called intermediatecalled intermediate--energyenergyneutronsneutrons..

C. FastC. Fast NeutronsNeutrons

These neutrons coverthe energyrange from 10These neutrons coverthe energyrange from 10 keVkeVupward.upward.


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2. NEUTRON SOURCES2. NEUTRON SOURCES

The most widely available neutron sources are nuclearThe most widely available neutron sources are nuclearfission reactors, acceleratorsfission reactors, accelerators, and, and radioactive sourcesradioactive sources..

FissionlikeFissionlike spectra have average neutron energiesspectra have average neutron energies

around 2 MeV, and arearound 2 MeV, and are available fromavailable from nuclearnuclearreactors,reactors,252252CfradioactiveCfradioactive (spontaneous fission) sources(spontaneous fission) sources,, criticalcriticalassemblies, and other mock fission sources such asassemblies, and other mock fission sources such as

thatproduced bythatproduced by 1212--MeVMeV cyclotroncyclotron--acceleratedaccelerated

protons on a thick Be targetprotons on a thick Be target.. FigureFigure 11 showsshows several suchseveral such spectra,the Watt spectrumspectra,the Watt spectrum

being an idealized shape forbeing an idealized shape for unmoderatedunmoderated reactorreactorneutronsneutrons..


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FIGURE 1

Fission-

neutron

spectra from

reactors

(Watt

spectrum)


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and 12.9and 12.9--15.6 MeV forT(d, n)15.6 MeV forT(d, n) 44He, depending on theHe, depending on theangle of neutron emission, incomingparticle energy,angle of neutron emission, incomingparticle energy,

and targetthickness, as shown in Figs. 3.a and 3.b.and targetthickness, as shown in Figs. 3.a and 3.b.

FIGURE 2. Neutron spectra for three Be(, n)sources: 241Am, 210Po, and 239Pu.


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FIGURE 3.a.Neutron energy vs. angle for

the D(d,n) 3He reaction.

FIGURE 3.b.

Neutron energy vs. angle

for the T(d, n)4H

e reaction.


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Cyclotrons can be used to produce neutron beams byCyclotrons can be used to produce neutron beams byacceleratingprotons oracceleratingprotons or deuterons intodeuterons into various targets,various targets,most commonly berylliummost commonly beryllium..

FigureFigure 44 shows theshows the typical belltypical bell--shapedshaped neutron spectraneutron spectra

thatresult from deuteron bombardmentthatresult from deuteron bombardment..

TheThe neutron energiesneutron energies extend from zero to somewhatextend from zero to somewhatabove the deuteron energy, and haveabove the deuteron energy, and have an averagean average ofofabout 0.4 times that energyabout 0.4 times that energy..

The neutron beamThe neutron beam becomes narrowerbecomes narrower and moreand more

forwardforward--directed as the deuteron energy is increased.directed as the deuteron energy is increased.


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FIGURE 4. Neutron spectra on the beam axis generated by 15-, 20-,

and24

-MeV deuterons striking a thickB

e target.


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3. Neutrons Dosimeters3. Neutrons Dosimeters

3.13.1-- Dosimeters with Comparable Neutron andDosimeters with Comparable Neutron and --RayRaySensitivitiesSensitivities

3.1.13.1.1-- AA--150 TE Plastic Ion150 TE Plastic Ion Chambers (B/A=1)Chambers (B/A=1)

Anyhydrogenous dosimeterthatresponds to theAnyhydrogenous dosimeterthatresponds to the

absorbed dose deposited byabsorbed dose deposited bythe protonsthe protons set in motionset in motionby neutron elasticby neutron elastic--scattering events will havescattering events will havecomparablecomparable neutron andneutron and --ray sensitivities.ray sensitivities.

However,the premier instrumentHowever,the premier instrument ofthisofthis type is thetype is thetissuetissue--equivalentplastic ion chambermade of Aequivalentplastic ion chambermade of A--150150TE plasticTE plastic..

This plasticThis plastic and several othertissueand several othertissue-- and airand air--equivalentequivalent

plastics were first devisedplastics were first devised byby ShonkaShonka et al. (1959).et al. (1959).


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The general equation forthe response of a dosimetertoThe general equation forthe response of a dosimetertoa mixed field ofa mixed field of neutrons andneutrons and --raysrays can be most simplycan be most simplywritten in thewritten in the formform

oror alternativelyalternatively asas

WhereWhere QQnn,,==total response (i.e.,readingtotal response (i.e.,reading--for example,thefor example,thecharge producedcharge produced in anin an ion chamber orthe light outpution chamber orthe light output

measured from a TLD) due tomeasured from a TLD) due to the combinedthe combined effects ofeffects ofthethe --raysrays and neutronsand neutrons,,

A =response per unit of absorbed dose in tissue forA =response per unit of absorbed dose in tissue for --

raysrays,,


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B =response per unit of absorbed dose in tissue forB =response per unit of absorbed dose in tissue forneutrons,neutrons,

DD = y= y--ray absorbed dose in tissue, andray absorbed dose in tissue, and

DDnn = neutron absorbed dose in tissue= neutron absorbed dose in tissue..

AA-- 150 plastic is used to manufacture ion chambers of150 plastic is used to manufacture ion chambers ofseveral sizesseveral sizes commercially.commercially.

Ithas a composition that closelymatches muscleIthas a composition that closelymatches muscle tissuetissueinin contentcontent ofhydrogenofhydrogen (10.2% by weight) and nitrogen(10.2% by weight) and nitrogen(3.5%), butthe carbon(3.5%), butthe carbon content iscontent is 76.1 % in place of76.1 % in place of12.3 %, and there is only 5.2 % oxygen in the plastic12.3 %, and there is only 5.2 % oxygen in the plasticinstead ofinstead of 72.9%.72.9%.


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Fortunately,theFortunately,the kermakerma factors forfactors forCCandand OO arearesufficiently similar,sufficiently similar, and thatand that ofH is so much larger,thatofH is so much larger,thatthethe kermakerma factor for Afactor for A-- 150 plastic is only a few150 plastic is only a fewpercentpercentgreatergreaterthan that formuscle atmost neutronthan that formuscle atmost neutron energies,energies,in spitein spite ofofthis gross difference inthis gross difference in composition.composition.

3.1.23.1.2 Rossi TE ProportionalRossi TE Proportional Counter (B/A=1)Counter (B/A=1)

InIn principle this device, as usually constructed withprinciple this device, as usually constructed with

tissuetissue--equivalentplastic wallsequivalentplastic walls, can, can be operated as an ionbe operated as an ion

chamber at full atmospheric orreduced pressurechamber at full atmospheric orreduced pressure..

HoweverHowever, it, it is usually operated as a proportionalis usually operated as a proportionalcounter.counter.


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3.23.2-- Neutron Dosimeters Insensitive toNeutron Dosimeters Insensitive to --RaysRays

(A(A


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where C(T,t) =measured value ofthe specific ywhere C(T,t) =measured value ofthe specific y--rayraycountingrate (c/s percountingrate (c/s per gram of foil),gram of foil),

= overall counting efficiency (counts per= overall counting efficiency (counts per disintegrationdisintegration))

including geometric effects,including geometric effects,A'(T,t ) = specific activity ofthe target atoms in the foilA'(T,t ) = specific activity ofthe target atoms in the foil(disintegrations(disintegrations ss--11 perper gram of foilgram of foil),),

tt= duration ofthe neutron irradiation (s= duration ofthe neutron irradiation (s),),

A = decay constantA = decay constant (s(s--11),),

T= delay after irradiation before counting (sT= delay after irradiation before counting (s),),

NNtt = number oftarget atoms per gram of foil= number oftarget atoms per gram of foil,,


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(E) = activation cross section oftarget atoms for(E) = activation cross section oftarget atoms forneutrons of energy E (cmneutrons of energy E (cm22/atom), and/atom), and

(E)= differential neutron flux density (cm(E)= differential neutron flux density (cm--22ss--11MeVMeV--11).).

Forthermal neutronsForthermal neutrons (E) has a fixed value(E) has a fixed value , and can, and canbe thus moved outside ofthe integral sign.be thus moved outside ofthe integral sign.

FastFast--neutron activation detectors exhibit energyneutron activation detectors exhibit energythresholds below whichtheir cross sections decrease tothresholds below whichtheir cross sections decrease tozero more or less steeply.zero more or less steeply.

A set of foils with differentthresholds allowsA set of foils with differentthresholds allows

determination ofthe spectrum of neutron flux density bydetermination ofthe spectrum of neutron flux density byan iterative computerprogram such as SAND II.an iterative computerprogram such as SAND II.


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3.2.23.2.2-- Fission Foils (A = 0Fission Foils (A = 0))

Fissionable activation foils were proposed byHurst etFissionable activation foils were proposed byHurst et

al. (1956) forpersonnelal. (1956) forpersonnel monitoring inmonitoring in fission neutronfission neutronfields, because ofthe lowthreshold energies ofthesefields, because ofthe lowthreshold energies ofthesefoilsfoils, shown, shown in Figs.in Figs. 5.a and 5.b.5.a and 5.b.

This method makes use of foils ofThis method makes use of foils of239239Pu,Pu,237237Np, andNp, and238238UU ..

TheThe 239239PuPu foil must be enclosed in a boron sphericalfoil must be enclosed in a boron sphericalshell 1shell 1--22 g/cmg/cm22 thicktothickto stop neutrons below about 10stop neutrons below about 10keVkeV,thus creating an artificial threshold,thus creating an artificial threshold..

This method is seriouslyhandicapped byrequiringThis method is seriouslyhandicapped byrequiringgram quantities ofgram quantities of fissionablefissionable materials thatrequirematerials thatrequirelicensing and careful controllicensing and careful control,, in addition to the activityin addition to the activity

measurements requiringmeasurements requiring decay corrections.decay corrections.


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FIGURE 5.a. Fission cross section of different fast-neutron

threshold detectors as a function of neutron energy.


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FIGURE 5.b. Effective fission cross section of 235U and 239Pu,

encapsulated in 1.65 g/cm2 of 10B, as a function of neutron energy.

radiation detection 2

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