Neutrons in radiation protection neutron shielding neutron dosimetry

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Neutrons in radiation protectionneutron
shieldingneutron dosimetry

• Prof. François Tondeur, DrSc
• ISIB, Brussels, Belgium

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Neutrons sources

• Reactors chain reaction of fission ? fast n
• thermal reactors thermal neutrons (1.5 kT)
• Accelerators various reactions
• e ? X and (X,n)
• (p,n), (p,xn) , .
• unwanted or applied (neutron therapy,)
• Generally fast neutrons
• Neutron generators, isotopic sources Am-Be, 252Cf
  for various applications fast neutrons
• Most n sources also produce g rays

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Neutron shields basic principle

• Fast neutrons interact mostly by scattering
• maximum energy loss with 1H(n,n)1H
• Slow neutrons are easily captured by
• 10B(n,a)7Li 6Li(n,a)3H
• 113Cd(n,g)114Cd ..
• (n,a) preferred (g need extra Pb shield)
• Two steps
• Slow-down of fast n by H-rich medium
• Capture of slow n by B

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Practice basics

• Water (d1/10?20-40 cm according to e) boric
  acid
• Easy to prepare
• Storage of source with easy handling
• Risk of leakage and loss
• Paraffin borate
• Easy to prepare
• Risk of fire/melting
• Concrete boron compound
• Increased thickness (x2), more weight
• Permanent even if fire

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Direct absorption

• Fast n shielding can also be based on direct
  absorption without moderation by (n,a) reactions,
  e.g. in steel
• Thickness a bit smaller than paraffin, much
  higher weight

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Neutron dosimetry
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Effective dose

• Effective dose E S wt.wr.Dtr Sv
• regulated (workers lt20 mSv/y, public lt1 mSv/y)
• sum over irradiated tissues t
• sum over radiation types r (e, g, p, n, a,
  .)
• D absorbed dose GyJ/kg
• physical quantity
• can be measured directly (ion chambers, )
• can be calculated from fluence F D C(e).F
  
• F fluence (particles/m2) eparticle energy

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Equivalent dose

• Htr wr.Dtr Sv
• Gammas and electrons wr1
• Neutrons wr(en)
• lt10 keV 5
• 10-100 keV 10
• 100-2000 keV 20
• 2-20 MeV 10
• gt20 MeV 5
• ? need of spectrometric information

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ICRU dose

• H and HP defined for measurement of the dose
  from penetrating radiation
• Defined for the normalization of devices
• Under specified test conditions, the devices must
  reproduce H Q.D calculated at 1 cm depth in
  the body
• Q(LET) depends on LET de/dx of (secondary)
  charged particles (eparticle energy)
• ? information on LET needed
• LET(e)

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Neutron-gamma discrimination

• n and g are both penetrating
• They are both indirectly ionising
• Evaluate LET of charged particles individual
  events
• g ? e low LET de/dx
• n ? p or nuclei , high LET
• Select specific reaction for n appropriate
  medium

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Thermal neutrons

• ? From dose measurements Hn 5 Dn .
• n/g difference of 2 detectors
• Thermoluminescence 7LiF (g only) , 6LiF (n
  g)
• irradiated LiF emits light when heated
• number of photons proportional to D
• ? From fluence / flux measurement
• BF3 , 3He counters
• n/g by pulse height (e range ltlt counter ? low
  energy deposited ? small pulses)

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Fast neutrons

• Fast neutrons interact in tissues mostly by
  elastic scattering on protons (80 of the dose)
• Tissue equivalent device high proportion of H.
• organic material, methane, hydrogen.
• n/g
• 2 detectors with and without H (e.g. CH4 / CO2)
• By pulse height in gas proportional counter
• By pulse shape in some organic
• scintillators
• e short pulse , p long pulse

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Tissue equivalent proportional counter

• Developed for cosmic rays
• Appropriate for high energies
• No discrimination
• Range gt detector even
• for secondary nuclei
• LET ? e / d ,
• d average track length
• in the detector

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Spectral measurement

• If the spectral response of the detector is
  known, usually by Monte Carlo simulation, and n/g
  discrimination possible, the pulse height
  spectrum can be unfolded
• R(En,Ed) pulse height (Ed) spectrum for
  neutrons of energy En to be calculated
• M(Ed) measured pulse height spectrum
• S(En) unfolded neutron fluence energy spectrum
  dF/de
• M(Ed)S R(En,Ed).S(En) or (M)(R).(S)
• (S) (R)-1.(M) deconvolution

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unfolding

• Measured spectrum and
• reconstructed one ?
• Matrix product not accurate
• due to the statistical errors
• present in the measured spectrum
• Search of best fit between the
• reconstructed spectrum
• (M)(R).(S) and the measured one , when varying
  (S)
• - unfolded spectrum ?

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Monitors with moderator

• Except for H-rich detectors, excessive
  sensitivity to slow neutrons (high s), nearly no
  sensitivity for fast neutrons
• ? moderator shield (e.g. PE) around detector
• partial absorption of slow neutrons (efficiency
  ?)
• slow-down of fast neutrons (efficiency ?)
• thickness adjusted ? same ratio H/N (Ncounts)
  for slow and fast
• but H/N too big for
• intermediate neutrons
• (H overestimated)
• Rem-meter

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Albedo dosimetry

• Principle use the human body as a moderator
• 2 6LiF detectors ( 2 7LiF for g)
• One shielded by Cd for slow neutrons from the
  body
• only sensitive to the slow neutrons of the field
• One shielded for slow neutrons of the field
• only sensitive to slow neutrons from the body
• fast neutrons from the field that are slowed
  down by the body
• ? calibration for slow and fast neutrons
• Not calibrated at intermediate energies

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Multi-sphere dosimetry

• Bonner spheres
• K moderator spheres i of increasing radius Ri
  around the detector
• K measurements of Ni counts allow to determine
  SkDF/De for K energy groups by deconvolution, if
  the response matrix is calculated
• Ni Sk R(i,k)Sk (N)(R).(S)
• ? (S)(R)-1.(N)
• Usually
• K10 or 12

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Bubble dosimeters

• Replace now albedo for personal dosimetry
• Superheated drops in a gel (at room T) are kept
  liquid by pressurisation . Pressure is released
  for the measurement
• Drops form bubbles when enough energy is
  deposited in them .
• This is the case for recoil protons (n), not for
  electrons (g)
• The design allows
• to approximately
• obtain Nbubbles?H
• Version for slow n with
• sensitive element (Li,B ?)