Alpha detector

Previous experience in the production of plutonium 238 revealed the need for the double alpha containment of the cells where the alpha-emitter isotopes with high specific activity are handled. Thus all the hot cells are equipped with a double ventilation system which provides ventilation of alpha-cells and ventilation between alpha-cells and biological shields. Alpha detectors continuously monitor the exhaust circuits. 

Effluent solution from the ion exchange column is passed through an in-line alpha-detector which activates a count-rate meter and recorder outside the cell. Inside the detector/ the liquid flow is passed adjacent to a Mylar film-covered window/ which separates the liquid from the silicon diode detector. 

A flow-through alpha detector in the column effluent line is used to monitor the progress of an elution, and a movable neutron probe is used to determine the location of the 252Cf... 

The separation column effluent is divided into about 15 fractions that are collected in small (250-mL) polyethylene bottles. The volume collected in each bottle is determined by the appearance of the alpha-emitting elements in the column effluent solution as indicated by the response from the flowthrough alpha detector a typical response curve is shown in Fig. 2. Normally/ two einsteinium fractions/ two intermediate fractions/ and three californium fractions are collected. The intermediate fractions are taken when the valley between the einsteinium and californium peaks occurs on the response curve and usually contain less than 5% of each element. Sometimes the alpha trace will show a small fermium peak just ahead of the einsteinium/ but usually there is not enough fermium alpha to make a response and the fermium is assumed to be in one or both of the two fractions taken just prior to the einsteinium. The berkelium is primarily a beta emitter and is not detected by either the alpha or neutron detectors thus, three fractions are usually taken after the californium alpha peak to isolate the berkelium. If there is a significant amount of 244Cm he feed (milligram quantities)/ the alpha trace will show a third major peak when americium and curium are eluted at the end of the run. 

Monitoring of the 232Cf band movement with the neutron probe (Fig. 3) provides advance information on when to expect einsteinium in the eluate and can be used as a backup indicator for changing effluent collection bottles during the einsteinium-californium elution if the alpha detector fails. Also/ the neutron probe can be used to detect problems such as insufficient resin in the columns or incomplete elution from the loading column. 

Figure 2. Alpha activity in the effluent from the separation column as measured by the flow-through alpha detector during a typical run...

After separation and purification, the pure fractions of uranium and plutonium are electroplated on polished stainless discs and the activities of their radionuclides measured using alpha spectrometry. The distribution value of alpha detectors, which is between 17 and 20 kev, is very important. Two radionuclides ( Pu and " °Pu) are measured together because the difference between the energy of their alpha particles is less than 15 keV [1, 14]. Figures 15.3, 15.4, and 15.5 present typical spectra for the alpha measurement of polonium, uranium, and plutonium [ 1 ]. 

The scanning alpha detector SADSAC was constructed at Argonne National Laboratory and consisted of a proportional counter whose aperture was a slit of. 005" wide. The sample was placed on a movable table and advanced under the slit in a regular manner 0.010" at a time by a micrometer arrangement. A scaler and timing device completed the system. Counts were made at appropriate intervals and the total recorded as a function of position on the sample. 

Figure 12.16 Alpha particle X-ray spectrometer (APXS). To determine the elemental composition of rocks and soils of Martian surface, the rovers, which landed in 2004 on Mars, carry a deployment device that could be in contact with the rocks. The sensor head contains six Cm radioactive sources, six alpha detectors and one X-ray detector in the centre. The accumulation time is a few hours per sample analysed.

Energy Resolution and Response Function of Alpha Detectors... 

Sill, C. W. and Olson, D. G. 1970. Sources and prevention of recoil contamination of solid-state alpha detectors. Anal Chem 42, 1596-1607. 

Several techniques to measure air concentrations are outlined by Breslin (1980). Most of the techniques for measuring radon use the fact that both radon-222 and the short-lived daughters are alpha- emitting nuclides. The sample is collected and taken back to the laboratory for "alpha-counting" or an alpha-detector is taken to the field for on-site measurement. There are several ways to measure alpha decay. A scintillation flask is one of the oldest and most commonly used methods. The flask is equipped with valves which are lined with a phosphor (silver-activated zinc sulfide) and emit light flashes when bombarded with alpha particles. Other methods draw the air through a filter (or filters) for a variety of time intervals and then count the number of alpha-decays occurring on the filter. EPA (1986) and NCRP (1988) reports provide more in-depth discussions of these methods. 

Kim, K. H., Bai, J. H., Whang, J. H., Jun, I. S. 2008. Development of alpha detector module based on large area PIN photodiode. Journal of Nuclear Science and Technology Supplement 5 417-420. 

Note 2 The mixed Th/ Th planchet is worthless after the decay of Th. A useful standardization can be made with a planchet with 60dpmU electrodeposited on it (evaporate a weighed aliquot in a small Teflon beaker and follow electrodeposition procedure of Th Section 13.7.7). After ingrowth of Th to secular equilibrium with (several months), the planchet can be used for a check on the relative efficiencies of beta and alpha counters. Alpha detector efficiency determined this way for should also be applicable to but an independent calibration of the °Th spike is still required. 

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