Alpha energy counting efficiencies

Determine counting efficiency of the proportional detector in Step 5 for three 3,000-s periods to measure alpha particles and beta particles. Record in Data Table 7.2. Also perform overnight count (50,000 s) for alpha-particle spectral analysis of the planchet to identify the uranium isotopes and any other radionuclides and to determine their relative amounts from their alpha-particle energy spectra and record results in Data Table 7.2. Count alpha- and beta-particle background in proportional counter and alpha-particle spectral background in spectrometer for at least the same periods. 

At, = total activity of rPu tracer added to the samples, multiplied by fraction of alpha particles of that energy per disintegration, in Bq, e = counting efficiency... 

If the last extractant is also a scintillator such as ETRAC s STRONEX, the equilibrated organic phase can be counted directly in a beta-liquid-scintillation counter or in a PERALS spectrometer. The carboxylic acid is not colored and does not quench. The PERALS spectrometer provides better beta-energy resolution and has only slightly lower counting efficiency for betas and thus may offer some advantage if both Sr and Sr are required in the same sample. The PERALS spectrometer will provide better separation of the 0.546 MeV Sr, the 1.48 MeV Sr, and the 2.28 MeV Y. In addition, if radium is present, the pulse shape discrimination feature of the PERALS spectrometer can be used to reject the contribution from radium alphas. 

For the first three applications, a radionuclide- and mass-specific counting efficiency musf be selected. For the fourth application, a thin sample—below 2.5 mg/cm for alpha-particle counting—should be prepared so that efficiency values are similar af commonly encountered energies. For counting beta particles, the sample should not exceed 10 mg/cm. An intermediate-energy (e.g., 0.6-0.8 MeV Pmax) radionuclide standard provides reasonable efficiency estimates except that the activity of a radionuclide that emits only low-energy beta particles will be underestimated. 

Measuring alpha particles with an LS counter is an attractive option because the counting efficiency is near 100% and no self-absorption problem exists. After the usual sequence of separations for radionuclides such as thorium, uranium, and transuranium isotopes, the radionuclide is prepared in the final solution for counting and yield determination. A tracer that emits alpha particles at a sufficiently distinct energy is added initially to measure yield. The factor that controls detection sensitivity is the background, typically of 1-2 c/m in the alpha-particle energy region of the LS counter. 

Although gamma rays are much less subject to attenuation than alpha and beta particles, a density correction is needed if the density of the sample deviates significantly from the density of the calibration standards. The effect of density on self-absorption for both the standard and the sample is estimated by Eq. (7.2) [x for this purpose is the photon attenuation coefficient in cm /g and x is the sample area density in g/cm. Values for ix in some common materials are listed in Table 2.2 and in its cited reference. If a large set of samples with consistent density is analyzed, it may be possible to prepare radioactivity standards at the same density to avoid the need for correction. Interpolating efficiency values as a function of density is feasible at energies above 0.1 MeV because the effect of minor density difference on counting efficiency is small. 

Computation of activity is simplified because the same counting efficiency applies to all alpha particles in the usual energy range. The activity of a radionuclide is calculated simply from the activity of the added tracer multiplied by the net accumulated counts for the peak of the radionuclide of interest and divided by the net counts for the tracer peak. Separate values of the counting efficiency and the yield are not needed, although they may be of interest to monitor tracer activity and process yield, respectively. 

Therefore, the probability of detecting an event is 1-0.01, or 99%. The specific ionization in air is more than 50 ion pair per cm for beta particles and about 400 times as great a value for alpha particles. The specific ionization in the filling gas is similar. Hence, a travel distance of 0.1 cm will exceed 99% intrinsic efficiency for counting both alpha and beta particles in the typical proportional counter, although most particles deposit only a fraction of their energy in the gas. 

Alpha spectrometry is characterized by good isotope separation, uniform (i.e., energy-and isotope-independent) detector efficiency and a very low backgroimd (typically of the order of 0.001-0.003 cpm (counts per minute) for the 4-8 MeV energy range of a new surface barrier detector, increasing with time as a result of the accumulation of recoil products from measured samples). It allows precise measurements at low activities, and easy calibration... 

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