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Radioisotope Detection

Radioisotope Detection. Stanley Glaros. Introduction.

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Radioisotope Detection

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  1. Radioisotope Detection Stanley Glaros Gull Group

  2. Introduction • The intent of this presentation is to provide a basis for detecting terrorist radiological threats. The threat must be detected by both first responders and fixed platform lab interrogated systems which must be verified before involving NEST. The best link is a dedicated internet link to the national labs tasked to evaluate the isotope from background to threat. Gull Group

  3. Topics of Discussion • We must be able to discriminate between threat and background. There is natural radioactivity from elements such as tritium H3, Be7, • C14, K40, Rb87 and the naturally occuring elements above atomic number 84 (polonium) which are unstable hence break down emitting alphas or betas, and most of the time, gammas, to reach a more stable configuration. • Real threats will come from: • 1. Medical & Industrial isotopic sources • 2. Reactor waste products (spent fuel) • 3. Weapons grade material & weapons Gull Group

  4. Medical & Industrial Sources • The medical isotopic sources easily obtained are Ga67, Tc99, Pd103, In111, I 131, Xe133, Tl 201. The industrial isotopic sources easily obtained are K40, Co57, Co60, Ba133, Cs137, Bi207, Po210, Ra226, Th232, Am241. • The best detection of these isotopes, is through their x-ray signatures since their beta & alpha emissions are easily masked with thin layers of material. Gull Group

  5. X-Ray Spectroscopy • Ionization Chambers, Proportional Counters, Geiger-Mueller Tubes, Semiconductor Photodiodes, and Scintillator/Photomultiplier detectors all listed below have the ability to give some information about the threat but only the last two can supply spectral information that will identify the isotope. Gull Group

  6. Semiconductor Detectors • Si-PIN, CdTe, CdZnTe, GaN, GaAs, Ge:Li (LN cooled) all successfully detect the spectra. The most precise spectra are measured with the Ge:Li which is cooled with liquid nitrogen to eliminate noise & broadening. The sensitivity and efficiency of the larger band-gap materials (CdZnTe, GaN) is approaching • 20%. Gull Group

  7. Scintillator/Photomulti-plier Detectors • The scintillator must be shielded with a Cd or B filled poly shield if there are neutrons present or suspected thus preventing their detection simultaneously. Scintillator materials vary from liquids such as NE213 solution bearing a phosphorus compound, to solids such as Pilot B, NE111, CsI, and NaI(Tl), coupled to a photomultiplier tube which sees the light generated as the x-ray exictes the atoms in the media. The sensitivity is limited to the volume of scintillator, size & gain of the photomultiplier. Gull Group

  8. Fission Fragments • Fission fragments contained in spent fuel rods represent a significant threat when dispersed in a high explosive matrix. Most fuel rods are fabricated of zirconium alloy (sometimes stainless steel) and contain small cylinders or pellets of U235 enriched uranium oxide. The spent rods contain some radioactive gases such as argon & xenon, some Pu239, Pu241, daughter products of the actinide/U238 series and fission fragments containing isotopes: • 1. Long half life:Sr90, Y90, Cs137, Ce144, Pr144, Nb95, Zr95, Y91, Ru106, Rh106 • 2.Medium half life:Sr89, Ce141, Ru103, Te129 • 3.Short half life:La140, Pr143, Te129, Ba140, I 131. Gull Group

  9. Weapons & Weapons Grade Material • Both gamma radiation from the decaying chain of fissile material and the two plus neutrons from each spontaneous fission are signatures which we can detect. The first has been discussed in the x-ray spectra section and the neutrons are detectable with five types of portable detectors: • 1.Shielded He3 or BF3 proportional counters • 2.Shielded scintillator/photomultipliers • 3.Shielded fission based detectors • 4.Shielded Proton Recoil Semiconductor • 5.Folded High Band Gap Semiconductor Gull Group

  10. Specific Weapon X-Ray Spectra • For the best use of resources, we should concentrate on U235 based weapons x-ray emission, but it normally is only 54% at 185 kev. This is easily shielded with 1-3 cm of lead. A promising impurity is U232 which though low in concentration, gives off a 2.614 Mev gamma. Using 8” (20cm) thick, 24”x24”(3716cm2) NaI, 100% of under 1 Mev photons will be detected, 97% of 1.33 Mev Co60, and 91% of 2.614 Mev U232 photons will be detected. Gull Group

  11. Proton Recoil NeutronDetection • Again shielded, there is a polyethylene converter layer nested on the surface of a photodiode structure where the interaction driven proton, interacts with the Si or GaAs. This response can be enhanced for low neutron fluences, with high voltage bias such as to promote avalanching but causing ensuing saturation. Gull Group

  12. Fission Enhanced Neutron Detectors • This type detection uses the fission of fissile coated cathodes, to produce 235 primary electrons (U235) while the 95% of the fission’s 206 Mev is released as kinetic energy in the fission fragments which in turn produce another 400 secondary ion-electron pairs which is a significant amplification gain of 600. We can use a moderator to slow down the neutrons to enhance th fission cross section in the fissile such as U235 or deal with normal 14 Mev neutrons interacting with insensitive U238 (more toxic Pu239 has the widest cross section). High voltage can enhance gain. This detector is vacuum sealed. Gull Group

  13. X-Ray Shielded Scintillator/Photo-multiplier Neutrons • In the case of sensitive measurement of low neutron fluences, one must use significant high z shielding to prevent gamma interaction. The most widely used system for larger arrays necessary for low statistics, involves NaI. The • Photomultiplier itself has issues of photocathode area, number of dynodes, operating voltage, time response, all leading to efficiency decisions. Even the multichannel analyzer has tradeoffs of channel width, number of channels, discrimination, etc. The data library base is best maintained at the national labs and internet accessed for best results in comparison. Gull Group

  14. Range • Specific ionization of an alpha (4amu) in air will cause between 50K-100K ion-electron pairs/cm whereas a beta (0.00055amu) of the same energy will cause between 30-300 ion-electron pairs/cm. The effective range for a beta of the same energy is 1000 times greater than the alpha’s. The attenuation of x-ray photons in lead is six times that in aluminum. Gull Group

  15. Activity • Isotope Decay Half Life Range air • H3 beta 12.3 yrs • Be7 beta 53.38 days • C14 beta 5730 yrs • Rb87 beta 4.7x10E9yr • Pb206 stable • Pb210 beta 22.3 yrs • Po210 alpha 138 days • Pb214 gamma 26.8 min • Ra226 alpha 1600 yrs • Th232 alpha 1.39x10E10yr 2.5cm • Th233 beta 23.5 min • U233 alpha 1.62x10E5yr 3.3cm • U235 alpha 7.13x10E8yr 2.9 • U238 alpha 4.51x10E9yr 2.7 • Pu239 alpha 2.44x10E4yr 3.6 Gull Group

  16. Radiation Units • 1 Becquerel=1 disintegration/sec • 1 Roentgen=1esu ion-elects/cm2 • =71000 Mev/cm3 • =88 ergs/gm air • 1 rad=absorbing 100ergs/gm in air • 1 Gray=1 joule/kg absorbed • =1 Sievert in tissue Gull Group

  17. Shielding & Range in Air • Shielding of the x-rays involve higher Z materials (Pb) as the x-ray becomes more energetic. A 1 kg Co60 source used in an obsolete x-ray irradiation facility will produce 4.19x10E16 becquerels (decays/sec). One cm of lead will allow transmission of 54.3% of the x-rays. • The transmission through 10m of air, of 662 kev x-rays, is 90%. For 1.33 Mev x-ray photons, it is 92%. For 2.61 Mev photons, it is 95%. The total absorption in air takes place by 1 km for 662 kev photons, 1.5 km for 1.33 Mev, and 2 km for 2.61 Mev. Gull Group

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