Neutrons in radiation protection neutron shielding neutron dosimetry

1. Neutrons in radiation protectionneutron shieldingneutron dosimetry Prof. Fran�ois Tondeur, DrSc ISIB, Brussels, Belgium

2. 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

3. 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

4. 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

5. 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

6. Neutron dosimetry

7. Effective dose Effective dose E = S wt.wr.Dtr [Sv] regulated (workers <20 mSv/y, public <1 mSv/y) sum over irradiated tissues t sum over radiation types r (=e, g, p, n, a, �.) D = absorbed dose [Gy=J/kg]: = physical quantity can be measured directly (ion chambers, �) can be calculated from fluence F: D = C(e).F F = fluence (particles/m2) e=particle energy

8. Equivalent dose Htr = wr.Dtr [Sv] Gammas and electrons : wr=1 Neutrons : wr(en) <10 keV 5 10-100 keV 10 100-2000 keV 20 2-20 MeV 10 >20 MeV 5 ? need of spectrometric information

9. 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 (e=particle energy) ? information on LET needed LET(e)

10. 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

11. 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 << counter ? low energy deposited ? small pulses)

12. 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

13. Tissue equivalent proportional counter Developed for cosmic rays Appropriate for high energies No discrimination Range > detector even for secondary nuclei LET ? e / d , d = average track length in the detector

14. 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

15. unfolding

16. 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 (N=counts) for slow and fast but H/N too big for intermediate neutrons (H overestimated) Rem-meter

17. 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

18. Multi-sphere dosimetry Bonner spheres K moderator spheres #i of increasing radius Ri around the detector K measurements of Ni counts allow to determine Sk=DF/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 K=10 or 12

19. 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 ?)