neutron radiation and dosimetry vylet

8/3/2019 Neutron Radiation and Dosimetry Vylet

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Neutron Irradiations and

Dosimetry

Vashek Vylet, PhD

Duke University Medical Center

Center for Medical Countermeasures Against Radiation


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Goal: Introduce you to

Challenges in Neutron Dosimetry

How we can determine dosimetricquantities of interest

Neutron irradiations available at Duke


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Facts and Challenges

Neutrons ionize indirectly, via secondary

charged particles: protons and heavier cp

Neutron energies span many decades

Their biological effects vary greatly withenergy


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10-2 10-1 100 101 102 103 104 105 106 107

En / eV

0.1

0.2

0.3

0.4

0.5

En

*

E(En

)/cm-2

10-2 10-1 100 101 102 103 104 105 106 107

En / eV

0.1

0.2

0.3

0.4

0.5

En

*E

(En

)/c

m-2

Soft Reactor Spectrum

Hard Reactor Spectrum

252Cf-Bare

D2

0 Moderated 252Cf

Example of Neutron Spectra


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Quantities (quick recap) Fluence: = dN/da [cm-2] ; dNis number of

particles impinging on a sphere around point ofinterest, with great-circle area da (particles/cm2)

Exposure X [Roentgen] Obsolete, not forneutrons; replaced by Kerma in air

Kerma K= dtr/dm[Gy=J.kg-1] or [rad] wheretr is energy tranferred by indirectly ionizingradiation (neutrons, )


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Quantities Absorbed Dose D = d/dm[Gy= J.kg-1 =

100 rad] where is energy imparted (to a smallvolume of mass dm)

Dose Equivalent H = D.Q [Sv=J.kg-1]where Q=f(LET)is the quality factor

Linear Energy Transfer: LET[keV.m-1] howdensely is energy imparted; much higher forprotons than electrons


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N vs : Biological effects

For same energy deposited, neutrons

much more effective (~10x) in damagingcells

Neutron secondaries: high LET (mostly p+

) Photon products: low LET (e- and e+)

1 MeV e- range in H2O: 4.3 mm

1 MeV p+ range in H2O: 0.023 mm

Ionization density much higher for p+


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Quantities

Equivalent Dose HT=wRDR [Sv=J.kg-1]

Effective Dose E=wTHT [Sv=J.kg-1

]

Dose-Equivalent Index HI[Sv=J.kg-1] i.e. max. Dose-Equivalent in an

ICRU tissue sphere (30 cm diameter).


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Quantities Fluence, Abs. Dose and Kerma are purely

physical = measurable quantities

H, HT

, E, EDE must be estimated orcalculated from measured (E), D(LET),

Measurable (not really) quantity: AmbientDose Equivalent (similar to Dose-EquivalentIndex in 30 cm sphere)


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Underground Quantity Definitions

Exposure is a quantity that everybody can

measure, but nobody wants

Dose equivalent is a quantity that everybodywants, but nobody can measure

Ambient Dose Equivalent The dose equivalentreceived by a 30-cm diameter spherical man.ifhe werent there

Loosely after J. McDonald


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Cant measure H,

so measure ()And use this

conversion factor


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Conversion Factors Calculated for humans (not mice)

using Monte-Carlo codes andincreasingly complex phantoms

VIP-Man, based on theVisible Human Project

MIRD Phantoms


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Triangle Universities Nuclear Lab

Two areas for neutron irradiations in TUNL

N

N

NTOF

SNSA


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TUNL

Charge particle beams at TUNL

less than 500 eVEnergy spread

50 nAHeavy ions

500 nA3He and 4He2 Apolarized protons and deuterons

2 A pulsed and 5 A dc (d)unpolarized protons and deuterons

Maximum current on target

p, d, 3He, 4He and heavier ions (c)Particle types

DC to 2.5 MHz with 1.5 ns wide pulses (b)Beam pulse repetition rate

1.5 to 19.0 MeV (a)Nominal energy range

Performance SpecificationsParameter


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Neutron production at TUNL

Reactions: 2H(d,n)3He, 3H(p,n)3He,7Li(p,n)7Be

High-flux yield from protons or deuterons

on

9

Be: Dose-equivalent rates from5 micro-A deuterons on 9-Be target

En [MeV] Sv/h rem/h

0.5 1.13 112.73.2 7.13 713.3

8 2.84 283.8

14 0.12 12.3


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Neutron production at TUNL

Shielded Neutron Source Deuterium gas target


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TUNL Beam profile at Shielded Neutron Source

Area with circular collimator

Position (cm)

Horizontal

Vertical

Relative

Neutron

Flux


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Dosimetry Goals at TUNL Measure neutron fluence and its energy

distribution (E)

Establish the photon contamination ofneutron beams: DG

Measure (and calculate) distribution ofdose as a function of LET.


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Neutron Beam Characterization Time-Of-Flight measurements for energy

Long-counter for fluence

Bonner spheres for fluence and energy

Ionization Chambers for tissue Kerma TEPC for Dose as function of LET (micro-

dosimetry)

Monte Carlo calculations for specificphantom: spatial distribution of D(LET)


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Long Counter Secondary standard for neutron fluence

measurements


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Duke Bonner Spheres

30.48

PAPortable

MCAHV

A

25.420.32

14.28 cm

He-3counter

12.7x12.7cmscintillator (C11)

Measurements of primary and scattered

neutron spectra in room using spectra

unfolding technique


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Twin Ionization Chambers Tissue-equivalent and graphite


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Twin Chamber Technique There are no pure neutron fields, photon

(gamma) always present: Dtot = DN + DG

Goal: separate DN and DG using twoionization chambers (IC):

Tissue-equivalent IC (T): equally sensitive to

N and G Carbon IC (U): very low sensitivity to N


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Twin Chamber Technique Response of TE IC:

Response of graphite IC:

kT, hT sensitivity of TE IC to N and G, resp. kU, hU sensitivity of graphite IC to N and G,

respectively (formalism of AAPM Report No. 7, Protocol forNeutron Beam Dosimetry)

U U N U G R k D h D= +

T T N T G

R k D h D= +


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Twin Chamber Technique Then DN and DG can be easily obtained from

measured RT and RU:

U T T U U U U T N G

U T T U U T T U

h R h R k R k RD D

h k h k h k h k

= =


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TEPC Tissue-equivalent Proportional Counter:

measures Dose as function of LET


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Monte Carlo Calculations Predict contribution of scatter in experiment

Calculate energy deposition patterns ingreat detail, including spatial and energy

distributions of secondary charged particlesin specific small animal phantoms

Establish conversion fluence-to-dosefactors for mice or other small animals


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Suitable voxel-based phantoms maybe developed using data from micro-CT and NMR, or possibly importedfrom computer models developed forother purposes (Duke, ORNL).

MONTE CARLO

neutron radiation and dosimetry vylet

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