Introduction to Radiological Sciences Neutron Detectors. Theory of operation. Types of detectors Source calibration Survey for Dose

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1 Introduction to Radiological Sciences Neutron Detectors Neutron counting Theory of operation Slow neutrons Fast neutrons Types of detectors Source calibration Survey for Dose 2 Neutrons, what are they? Neutrons are a component of an atom Interestingly enough, NO radioisotopes spontaneously emit neutrons (exception: some very short lived and rare fission fragments emit neutrons) neutron

2 Neutrons, How are they made Fission (e.g. U-235, Cf-252) Cf-252 undergoes roughly 0 spontaneous fission per 33 alpha particles emitted. Cf-252 emits 2.3E6 neutrons per second per microgram. Fission neutrons have wide range of energies: Cf-252 most probable energy is MeV, average energy is 2.3 MeV Neutrons, how are they made Nuclear Reactions Better known as: Smashing things into an atom Example: Take a Beryllium ( Be) atom. Smash an 9 alpha particle into it. The Be absorbs the alpha ( 2He) and kicks out a neutron to make ( C) plus a free 6 neutron (n). ` 9 Be C n 5 Nuclear Reactions Americium, Radon, Plutonium, Polonium all have isotopes which emit alpha particles Therefore Mix them with Beryllium to make a neutron generator (e.g. Am-Be, Pu-Be) Theory: monoenergetic alphas make monoenergetic neutrons Reality: Alpha in mixed sources (e.g. Am-Be) most likely loose energy by colliding with other atoms before being absorbed by a Be. Therefore neutrons are emitted with spectrum of energies 6 2

3 UML 2Ci PuBe Source Spectrum at 60cm.00E+0.00E+03 Neutrons/cm2.00E+02.00E+0.00E+00.00E-0 fluence fluence/lethargy.00e-02.00e-03 Energy in MeV 7 Nuclear reaction Tidbit 5MV and 8MV Linear accelerators are used to treat cancer. Photons >0MeV may interact with material and can deposit enough energy to release neutrons. Therefore: Cancer treatment rooms for >0MV Linacs must be designed to protect against neutron exposure 8 Neutron energies Slow neutrons: determined to be below 0.5ev (cadmium cutoff) Absorption with a nuclei and nuclear induced nuclear reactions are the predominant interactions Can t be detected directly because of the slow neutrons small kinetic energy In a gas it takes 33.7ev/ion pair so it cant liberate electrons and a positive ion (ionize) Thermal neutrons have an energy of 0.025ev 9 3

4 Neutron energies Slow neutrons: (cont.) nuclear induced nuclear reactions Radiative capture (n,γ) is most probable but gamma rays are hard to detect but useful in shielding applications Radiative capture (n,α),(n,p), and (n,fissionfragments) Cross sections (interaction probabilities) can be looked up in the chart of the nuclides 0 Neutron energies Fast neutrons: determined to be above 0.5ev (cadmium cutoff) Scattering becomes of greater importance because the neutron can deliver an appreciable amount of energy to the recoil nuclei At every scattering site the neutron loses energy and is thereby moderated or slowed to a lower energy The most efficient moderator is hydrogen because elastic scatter is the most predominant interaction whereby the neutron can lose all its energy in one interaction (due to its similar Neutron energies Fast neutrons: (cont.) Inelastic scattering, For faster neutron energies, becomes possible with the nuclei in which the recoil nucleus is elevated to one of its excited states (become excited). The nucleus quickly de-excites emitting a gamma ray and the neutron loses a greater fraction of its energy that it would in an equivalent elastic scatter (this is key for high energy neutron shielding)

5 BF 3 neutron counter Boron lined proportional counters The interior of the detector wall is coated with boron to respond to thermal neutrons 0 7 n 0 thermal B Li BF 3 neutron counter Boron lined proportional counters are thermal detectors but when used with different sized moderators one can gather some neutron spectrum information. BF 3 Long counter Known as the flat response detector This counter is designed to be sensitive to neutrons incident on the end cylinder face Uses BF 3 detector where the outer cylinder is coated with boron not the end face of the cylinder 5 5

6 3 He neutron counters 3 He proportional counters Uses n,p reaction and the proportional counter measures the recoil proton 3 3 n He H p He Ionization chamber 3 He scintillator 3 He semiconductor 6 6 Li neutron counter Neutron scintillation counting Slow neutrons can be measured using a scintillator such as 6 LiI(Eu) crystal 6 3 n Li H Scintillators can be used in conjunction with hydrogenous substances to measure the proton recoil from the neutron interaction Types Lithium Iodide scintillators, Lithium glass scintillators, Lithium glass fiber scintillators, Lithium sandwich spectrometer can measure Gammas as well 7 Li is insensitive to neutrons and measures gamma only 7 Fission chamber Essentially an ion chamber coated on the inside with some fissionable material (i.e. U- 233, U-235, Pu-239) 235 n U 2FF n Q Due to the large Q (energy release or heat energy from the reaction) upwards of 200Mev meaning larger pulse than from the competing reaction for the fission fragments 8 6

7 Neutron Activation detection Activation counting relies on neutron activation of a material that then decays with some measurable activation product. Gadolinium activation counting Thermal neutrons capture of 255,000 barns (one of the highest cross sections in any material) Neutron absorption results in prompt reaction products such as gammas and conversion electrons These electrons are directly ionizing (therfore can be measured ) The electron of interest is the 72 kev 9 Neutron Activation detection Reaction Energy Range Detected particle 5 In(n,γ) 6 In Thermal B-, γ 97 Au(n,γ) 98 Au Thermal B-, γ 23 Na(n,γ) 2 Na Thermal B-, γ 32 S(n,p) 32 P >3Mev B- 27 Al(n,α) 2 Na >6Mev γ 27 Al(n,2pn) 22 Na >25Mev γ C(n,2n) C >20Mev B+, γ C(n,spall) 7 Be >30Mev γ 98 Hg(n,spall) 9 Tb >600Mev α,γ 97 Au(n,spall) 9 Tb >600Mev α,γ 20 7