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Photon-insensitive, thermal to fast neutron detector
Nuclear Instruments and Methods 179 (1981) 615-616 © North-Holland PubhshmgCompany
LETTERS TO THE EDITOR PHOTON-INSENSITIVE, THERMAL TO FAST NEUTRON DETECTOR Robert E APFEL Department oJ Engmeermg and Apphed Science, Yale Untverstty, Mason Lab, P 0 Box 2159, New Haven, CT 06520, U S A
Received 14 October 1980
Superheated drops of dlchloro-dlfluoro methane at room temperature and atmosphere pressure vaporize when exposed to thermal, intermediate, and fast neutrons A mechamsm for the thermal sensltlWty is proposed
sensmvlty Neither the neutron-carbon, nor the neutron-fluorine nucleus interaction provided the needed result, but the following reaction with chlorine does [2]
We have reported recently on the use of superheated drops m a gel as a fast neutron detector [1] Here we mention some recent tests with drops of dlchloro-dlfluoro methane (CC12F2 often going under the Dupont Co trademark, Freon 12) We knew that this detector was sensitive to fast neutrons, having observed frequent drop vaporization when a sample was exposed to a Pu-Be source and to nearly mono-energettc neutrons m the 2 - 4 MeV range produced using the (d, d) reaction on a 1 MeV Van de Graaff Accelerator (Sloane Physics Lab, Yale Unlv ) We were also aware that vaporizations would occur even when the Pu-Be source was m a one meter diameter water bath, suggesting either an unexpectedly high sensltw~ty to those fast neutrons that made it through the water moderator or a thermal neutron sensitivity Recent informal experiments at the National Bureau of Standards confirm the thermal sensltwlty When vaporizations were observed for neutrons having an energy as low as 85 keV (generated by the p + 7L1 reaction at the NBS Van de Graaff), we took our samples to the NBS Research Reactor and exposed samples first to a 2 keV filtered neutron beam and then to a very pure thermal beam (cad° mmm ratio for a 1/u detector ~104) In both cases reasonably frequent vaportzations were observed When a cadmium stueld was placed in front of the thermal beam (passing gammas and whatever few fast neutrons might be present, but suppressing the thermals), no reactions were observed m a waltmg tune about 30 tunes compared to the waiting tune for events without the shield Our next job was to determine what +Q nuclear reaction might be responsible for the thermal neutron
n + 3SCl-+ 3ss + p (0 6151 MeV) The cross section for this reaction is on the order of 0 4 b [3] Interestingly enough, this reaction mimics the following mtrogen reaction n + 14N --~ 14C +p (0 6264 MeV), which has a cross section of about 1 75 b and which is a prunary mechamsm by which thermal neutrons produce biological damage [4] Our observations of thermal, intermediate, and fast neutron senslUvity of superheated drop-in-gel compos~tlons, coupled with our earlier observations of photon msensitlvlty below 6 MeV, suggest apphcatlons of this composition to personnel doslmetry [5] More quantitative work, however, must precede any Final determination My thanks to Mark Roos, who performed many measurements, and to Oren Wasson, Kenneth Dural and Robert Schwartz of the National Bureau of Standards for providing the neutron sources (accelerator and reactor) which helped to confirm the thermal neutron sensitivity The work was supported by Heat Transfer Program, National Science Foundation
References [1] R E Apfel, Nucl Instr and Meth 162 (1979) 603 [2] C Maples, G W Goth and J Cerney, Nuclear Data (Sect A), 2 (1966) 473 615
616
R E Apfel / Thermal to fast neutron detector
[3] D I Garber and R R Kmsey, Neutron cross sections, Vol 2, Curves (Brookhaven Nat Lab, Upton, N Y , 3rd e d , 1976) p 89
[4] G S Hurst and J E Turner, (Wiley, New York, 1970) p [5] R E Apfel, Detector and other radiation, U S Patent
Elementary radmtlon physlcs 63 dosimeter for neutrons and No 4,143,274
LETTERS TO THE EDITOR PHOTON-INSENSITIVE, THERMAL TO FAST NEUTRON DETECTOR Robert E APFEL Department oJ Engmeermg and Apphed Science, Yale Untverstty, Mason Lab, P 0 Box 2159, New Haven, CT 06520, U S A
Received 14 October 1980
Superheated drops of dlchloro-dlfluoro methane at room temperature and atmosphere pressure vaporize when exposed to thermal, intermediate, and fast neutrons A mechamsm for the thermal sensltlWty is proposed
sensmvlty Neither the neutron-carbon, nor the neutron-fluorine nucleus interaction provided the needed result, but the following reaction with chlorine does [2]
We have reported recently on the use of superheated drops m a gel as a fast neutron detector [1] Here we mention some recent tests with drops of dlchloro-dlfluoro methane (CC12F2 often going under the Dupont Co trademark, Freon 12) We knew that this detector was sensitive to fast neutrons, having observed frequent drop vaporization when a sample was exposed to a Pu-Be source and to nearly mono-energettc neutrons m the 2 - 4 MeV range produced using the (d, d) reaction on a 1 MeV Van de Graaff Accelerator (Sloane Physics Lab, Yale Unlv ) We were also aware that vaporizations would occur even when the Pu-Be source was m a one meter diameter water bath, suggesting either an unexpectedly high sensltw~ty to those fast neutrons that made it through the water moderator or a thermal neutron sensitivity Recent informal experiments at the National Bureau of Standards confirm the thermal sensltwlty When vaporizations were observed for neutrons having an energy as low as 85 keV (generated by the p + 7L1 reaction at the NBS Van de Graaff), we took our samples to the NBS Research Reactor and exposed samples first to a 2 keV filtered neutron beam and then to a very pure thermal beam (cad° mmm ratio for a 1/u detector ~104) In both cases reasonably frequent vaportzations were observed When a cadmium stueld was placed in front of the thermal beam (passing gammas and whatever few fast neutrons might be present, but suppressing the thermals), no reactions were observed m a waltmg tune about 30 tunes compared to the waiting tune for events without the shield Our next job was to determine what +Q nuclear reaction might be responsible for the thermal neutron
n + 3SCl-+ 3ss + p (0 6151 MeV) The cross section for this reaction is on the order of 0 4 b [3] Interestingly enough, this reaction mimics the following mtrogen reaction n + 14N --~ 14C +p (0 6264 MeV), which has a cross section of about 1 75 b and which is a prunary mechamsm by which thermal neutrons produce biological damage [4] Our observations of thermal, intermediate, and fast neutron senslUvity of superheated drop-in-gel compos~tlons, coupled with our earlier observations of photon msensitlvlty below 6 MeV, suggest apphcatlons of this composition to personnel doslmetry [5] More quantitative work, however, must precede any Final determination My thanks to Mark Roos, who performed many measurements, and to Oren Wasson, Kenneth Dural and Robert Schwartz of the National Bureau of Standards for providing the neutron sources (accelerator and reactor) which helped to confirm the thermal neutron sensitivity The work was supported by Heat Transfer Program, National Science Foundation
References [1] R E Apfel, Nucl Instr and Meth 162 (1979) 603 [2] C Maples, G W Goth and J Cerney, Nuclear Data (Sect A), 2 (1966) 473 615
616
R E Apfel / Thermal to fast neutron detector
[3] D I Garber and R R Kmsey, Neutron cross sections, Vol 2, Curves (Brookhaven Nat Lab, Upton, N Y , 3rd e d , 1976) p 89
[4] G S Hurst and J E Turner, (Wiley, New York, 1970) p [5] R E Apfel, Detector and other radiation, U S Patent
Elementary radmtlon physlcs 63 dosimeter for neutrons and No 4,143,274