Beta-particle counters

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.

A special benefit of counting radiations in coincidence is that the process permits the absolute measurement of the activity of the radionuclide. For the beta-particle counting efficiency sp and the gamma-ray counting efficiency y (in count/disintegration), the activity A is related to the three count rates. The count rate Rp of beta particles in the beta-particle counter, the count rate Ry of gamma rays in the gamma-ray counter, and the coincidence count rate Rpy (in count/s) yield A (in Bq) by ... 

Lead-210 has a half-life of 20.4 years. This isotope decays by beta particle emission. A counter registers 1.3 X 104 disintegrations in five minutes. How many grams of Pb-210 are there ... 

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors. 

Liquid scintillation counter (low-energy beta particles) 9... 

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. 

Alpha-particle detector Beta-particle detector Gamma-ray detector proportional counters silicon (Si) diode with spectrometer proportional counters Geiger-Muller counters liquid scintillation (LS) counters thallium-activated sodium iodide (Nal(Tl) detector with spectrometer germanium (Ge) detector with spectrometer... 

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 2. Prepare sources in duplicate for the proportional counter by pipetting the appropriate volume - typically 100X - specified by the instructor of each of 3 standard solutions and the unknown solution onto the center of a separate planchet. You will have a total of 8 planchets 2 each for the beta-particle standards and the unknown beta-particle sample. Dry under heat lamps placed at sufficient distance over the planchets for slow drying. 

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. 

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. 

For each point calculate the counter efficiency for each sample. This is the factor that provides a value of the curves at x = 0 and f = 1.0. What is the average of all the counting efficiency values How does this value compare to measured counting efficiency values for beta particles of this energy in this type of detector ... 

Step 4. Disassemble the filtering apparatus and remove the filter with forceps. Fix the filter to a planchet with 2-sided tape. Count the sample three times with a proportional counter for alpha particles and beta particles for 3,000 s. Record mid-point of counting time. Record counting data in Data Table 7.1. Also measure detector background data for at least the same period and record in Data Table 7.1. 

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. 

Average proportional counter net a-particle and beta-particle rate (R) of three counts (alpha) cps (beta) cps... 

Step 7. Count the sample three times for 3,000 s each with a proportional counter for alpha and beta particles and record in Data Table 7.4. Record the time. 

Step 8. Evaporate 10 ml 0.5 M oxalic acid to about 2 mL and pour onto the second planchet with dried 100-A. sample of the initial uranium solution (see Part 7A, Step 5). Evaporate to dryness under the heat lamp. Flame the planchet as in Step 6. Count three times for 3,000 s each with proportional counter for alpha and beta particles. Record in Data Table 7.5... 

Scheme 2. Count the sample immediately with an a and (3 counter (e.g., the proportional counter) for 200 minutes. Repeat the count each day for 14 days or until the count rate equals or nearly equals the background. Obtain background counts for both alpha-particle and beta-particle counting modes. Subtract respective backgrounds for each count period and record in Data Table 8.7...

In this experiment, tritiated water is purified by simple distillation, and the tritium beta particles in the condensate are measured with a liquid scintillation (LS) counter. Such distillation also can collect tritiated water samples from solids. Tritium in other forms must be processed before it can be counted like tritium in water for example, tritiated hydrogen gas and tritiated organic substances can be oxidized to form water. Additional separations may be needed if the liquid or solid sample contains radioactive gases or volatile substances other than tritium that may be collected with the distilled tritiated water. Such radioactive impurities can be identified in the data output from the LS counter of an energy spectrum that differs from that of pure tritium, or of counts in energy regions where tritium counts are not found. 

Step 13. Count the sample for beta particles with proportional counter for 6,000 s and record results in Data Table 10.1 Measure the background count rate for 6,000 s immediately before or after the sample measurement. To check radioactive decay rate, repeat counting under identical conditions every second or third day for at least two weeks. 

Rutherford and his students used a screen coated with zinc sulfide to detect the arrival of alpha particles by the pinpoint scintillations of light they produce. That simple device has been developed into the modern scintillation counter. Instead of a ZnS screen, the modern scintillation counter uses a crystal of sodium iodide, in which a small fraction of the Na ions have been replaced by thallium (TH) ions. The crystal emits a pulse of light when it absorbs a beta particle or a gamma ray, and a photomultiplier tube detects and counts the light pulses. 

The Geiger counter (Fig. 19.4) consists of a cylindrical tube, usually of glass, coated internally with metal to provide a negative electrode and with a wire down the center for a positive electrode. The tube is filled to a total pressure of about 0.1 atm with a mixture of 90% argon and 10% ethyl alcohol vapor, and a potential difference of about 1000 V is applied across the electrodes. When a high-energy electron (beta particle) enters the tube, it produces positive ions and electrons. The light electrons are quickly accelerated toward the positively... 

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