Anticoincidence shielding

Radiometric analysis using a plastic scintillator connected to coincidence and anticoincidence circuits (e.g. a Picobeta counter) can be used to detect "Tc. The detection limit is estimated to be about 10 mBq. A slightly better detection limit (1-5 mBq) is reported for a gas flow counter with an anticoincidence shield [8],... 

Ballestra et al. [32] described a radiochemical measurement for determination of "technetium in rain, river, and seawater, which involved reduction to technetium (IV), followed by iron hydroxide precipitation and oxidation to the heptavalent state. Technetium (VII) was extracted with xylene and electrode-posited in sodium hydroxide solution. The radiochemical yield was determined by gamma counting on an anticoincidence shield GM-gas flow counter. The radiochemical yield of 50 to 150 litre water samples was 20-60%. 

Figure 2. The anticoincidence-shielded, high resolution, Ge(Li) gamma-ray spectrometer. The annulus is in position for inserting a large environmental sample...

To reduce interference from Compton scattering, an anticoincidence shield, 76 cm. X 76 cm., was constructed as shown in Figures 2 and 3. The shield consists of two independent type NE-102 plastic phosphor annuli. A 10-cm. bore through the top annulus accommodates the Ge(Li) detector chamber and the cryogenic assembly. The bottom annulus (i.d. diameter 25 cm.) houses a 20-cm. diameter by 15-cm. thick Nal(Tl) scintillator. Normally, the plastic phosphor is used in conjunction with the Nal(Tl) to form a well -shaped anticoincidence shield. Altema-... 

A drive mechanism raises the annulus for inserting the sample. Most samples are accommodated in a position just above the midline of the anticoincidence shield. The Ge(Li) detector is just above the sample a raise-lower mechanism provides approximately a 10-cm. vertical range in its position. The counting of small planchets does not require separation of the annuli a long plastic tongue may be used to insert the sample through a small door in the shield. 

Ge(Li) Detector Characteristics. Resolution measurements for the 18-cm.8 Ge(Li) detector were made with the anticoincidence shield in the inoperative mode, with a normal operating bias of 1700 volts, and with a preamplifier designed in our Laboratory (3, 4), and operated in conjunction with a Tennelec TC-200 linear amplifier. Resolution at 1.33 M.e.v. was 2.62 k.e.v., FWHM (Figure 4). The electronic pulser resolution for the amplifier system at a slightly lower energy was 1.86 k.e.v., the total capacitance of the detector was 28 pF, the noise slope was 0.035 k.e.v./pF, and the leakage current at 1700 volts was 0.5 X 10"9 amp. 

Left without anticoincidence shielding. Right with shielding. The presence ing. The sample was counted in 200 ml. of water in each... 

Left without anticoincidence shielding. Right with shielding. Note the... 

Figure 7. Spectrum from 100 pCi of 137Cs mixed with 0.5 fid of °Co in 200 ml. of water. The spectrometer was an 18-cm.3 Ge(Li) detector with anticoincidence shielding...

Anticoincidence, Coincidence Recording. An inherent problem in an anticoincidence-shielded spectrometer is that a decay scheme having gamma-rays in coincidence will appear to produce the same interaction between the Ge(Li) detector and anticoincidence shield as that of a Compton event. If only the events from the Ge(Li) detector are counted in the absence of a signal from the shield, those nuclides that decay... 

The use of anticoincidence shielding significantly reduces the Compton continuum and allows the detection of weak spectral lines usually masked by interfering Compton radiation. Further improvement resulted from separate recording of the coincidence and anticoincidence thus, radionuclides that normally decay with a coincidence scheme can be recorded without loss of efficiency. 

An anticoincidence-shielded y spectrometer utilizing a scintillation P detector is shown in Figures 8 and 9. The assembly consists of two... 

Figures 4 and 5 also indicate that a decay period of approximately 25-30 days reduces the intermediate-lived radionuclides such as 24Na, 140La, and 82Br to insignificant activity levels without seriously affecting the ability to measure the remaining isotopes of interest. The second count is then conducted for 100-1000 min on a Ge(Li) or anticoincidence shielded Ge(Li) spectrometer (28) and provides concentrations for Ag, Ba, Ce, Co, Cr, Cs, Eu, Fe, Hf, Hg, Ni, Rb, Sb, Sc, Se, Sr, Ta, Tb, Th, and Zn.

Natural Radioactivity Measurement. The naturally occurring radionuclides U(226Ra), Th(232Th), and K(40K) can also be determined on 100-g samples by direct counting of unirradiated samples using anticoincidence shielded multidimensional y-ray spectrometers (29, 30). These spectrometers use large (12 in. diameter X 8 in thick) principal Nal(Tl)... 

Figure 9. Anticoincidence-shielded Ge(Li) y-ray spectra of neutron-activated bottom ash...

Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

A gas proportional counter with an anticoincidence shielding counter has also been utilized. Ethane gas is commonly used as a counting gas. A water sample is reduced to hydrogen gas using Mg or Zn powder in a furnace. Hydrogen gas thus produced is added to dead ethylene or acetylene gas with a Pd catalyst to form ethane-counting gas. 

Figure 3, Anticoincidence shielded multidimensional gamma-ray spectrometer...

In order to further reduce the phoswhich background level, both a lateral and a top anticoincidence shields are provided. The later2d shielding system is made of 4 CsI(Na) scintillators 10 mm thick. The top shield is made of an organic scintillator 1 mm thick. 

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