Counting beta-particle

You are preparing to count beta particles from samples that contains low levels of "Mo. The final counting form is Mo03. Outline an experiment to prepare a "Mo beta self-absorption curve. How would the self-absorption be used to correct the observed counting activity in actual measurements (consider Tc-99m ingrowth) ... 

To determine the concentration of 131I in drinking water by purification sequentially as silver iodide and palladium iodide, followed by counting beta particles. 

To determine radio-strontium isotopes in ash or water samples by conventional radiochemical separation method and counting beta particles. 

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. 

The G-M counter is a simple and relatively inexpensive gas-filled tube with a count-rate meter an amplifier may also be present. The G-M detector counts alpha particles, beta particles, and gamma rays with a very thin window, counts beta particles and gamma rays with a thicker window, and counts gamma rays only with a thick shield. Alpha and beta particles interact in the gas gamma rays interact mostly in the walls, from which electrons enter the gas. The intrinsic efficiency for counting gamma rays relative to beta particles depends on the amount and type of solids surrounding the detection gas. 

Gross alpha and gross beta activity can be determined by various radioactive counters, such as internal proportional, alpha scintillation, and Geiger counters. Radium in water can be measured by co-precipitating with barium sulfate followed by counting alpha particles. Radium-226 can be measured from alpha counting of radon-222. Various methods are well documented (APHA, AWWA, and WEF 1998. Standard Methods for the Examination of Water and Wastewater, 20 ed. Washington DC American Public Health Association). 

Beta particle absorption Compound radioisotopic decay Radioisotopic decay Introduction to Geiger detectors Introduction to counting statistics... 

Shaw also describes two techniques that now make it possible to analyze particles without removing them from collection filters. To determine the mass of a sample, technicians insert the particle-laden filter between a source that emits beta particles and a detector that counts them. As the mass increases, the number of particles that can penetrate the sample decreases. To determine the atomic elements in a specimen, laboratory workers may also separately carry out x-ray fluorescence spectroscopy. X-rays passed through the sample cause each element to emit charactenstic x-rays. The energy levels of the rays reveal the identity of the elements the intensity of the x-rays (number emitted) reflects the concentrations. 

Beta particles at the deposition point passed through the Mylar tape and a 0.0075 cm thick A1 vacuum window and were counted with a 450 mm2 totally depleted surface barrier Si detector 1000 Mm thick. The geometric solid angle for a point source under these conditions was estimated to be 17, however, the measured efficiency was about 14. ... 

In Part 2A, the student will calibrate a gas-flow, end-window, anti-coincidence proportional counter for beta-particle counting efficiency as function of energy with certified standard solutions, and perform quality assurance (QA) counting tests. 

Calibration of a liquid scintillation detector for beta-particle counting is discussed in Experiment 9. 

The counting efficiency for the shown system approaches 52% for radionuclides with high maximum beta-particle energies (see Fig. 2A.2). This value exceeds the 39.6% based on the geometry of a 2.2-cm-dia. sample on a filter relative to the detector window. The counting efficiency exceeds the... 

Figure 2A.1 Cross-sectional view of a low-level anti-coincidence beta-particle counter A. Sample on a planchet. B. Thin window detector. C. Guard detector. Lead shielding surrounds the entire detector system. Typical background count rates are about 1 count per minute for beta particles and 0.1 count per minute for alpha particles. A sample mounted on a planchet (A) is placed below the thin window. When the guard detector (C) is triggered by an extraneous radiation that penetrates the lead shield, the sample detector (B) is inactivated. Immediately following, the detector (B) responds to beta particles from the sample. For low-activity samples, the probability is low that a particle from the sample registers a pulse at the same time that the counter is inactivated.

Figure 2A.2 Typical curve of beta-particle counting efficiency vs. energy for a thin-...

Beta particle calibration sources span energies from about 100 to 3,000 keV for proportional counters, and down to a few keV for liquid scintillation counters. In this experiment, a low-background, gas-flow, end-window proportional counter with automatic sample changer for alpha- and beta-particle counting is calibrated. Beta-particles sources are counted with pulse-height discrimination to eliminate interference from alpha particles the discriminator may be turned off when no alpha particles are present. 

Step 1. Place 10 blank planchets in the proportional counter system and count each for 50,000 s at settings (a) and (b) to determine the beta-particle background count rate. 

Step 3. Place in the counter sample changing system the planchets with the the beta-particle standard sources and the unknown beta-particle sample. Add two background planchets. Set the time control to count each sample for a time period specified by the instructor so that each accumulated count is at least 1,000 counts (typically, 500 s per sample, 50,000 s per background). Count each of the samples at settings (a) and (b). Repeat the count. Record your measurements in Data Table 2A.2. 

Step 4. Count one of the beta-particle standard sources 10 times, using setting (a). Record your measurements in Data Table 2A.3. 

Data Table 2A.3 Replicate Beta-particle Counts for Step 4... 

What is the difference between the beta particle count rate with and without pulse-height discrimination ... 

What is the significance of la, 2a, and 3a deviations reported with a numerical value For the beta-particle standard solution counted repeatedly, what percentage of the values is in the range of +la to -la ... 

How does the beta-particle-efficiency value (e), calculated from your data, compare to that used by the count room for the same counters If it is more than 5% higher or lower, give a reasonable explanation. 

The salt used in this experiment is potassium chloride (KC1). Potassium-40 is a naturally-occurring isotope of potassium (abundance = 0.0117%) whose half life is 1.28 x 109 years. It emits a gamma ray at 1.461 MeV with a decay fraction of 10.7%, in addition to beta particles with a decay fraction of 89.3%. The decay scheme for 40K is given in Fig. 3.1. A tared container (see Experiment 2) is filled with solid KC1, closed, weighed, and counted. 

This experiment examines the count rate as a function of sample thickness. All other variables are held constant (except for a small change in source-detector distance). As the sample becomes thicker, more of the beta particles are absorbed in the sample itself. This is called self-absorption, and is shown in Figure 4.1. In thin samples, self-absorption is relatively small or negligible, but in thick samples it is measurable and must be considered when calculating the counting efficiency. 

The formula for the self-absorption factor is exact for gamma rays (see Experiment 3) but approximate for beta particles. That it is applicable at all is due to the near-linear decrease of the logarithm of the count rate with absorber thickness of a beta-particle group (see Figure 2.6 in the Radioanalytical Chemistry textbook). The obvious deviation is that this relation ends at the range of the maximum-energy beta particle, whereas it continues indefinitely for gamma rays. 

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