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textbook:nrctextbook:chapter11 [2025-02-24 14:12] Merja Herzig |
textbook:nrctextbook:chapter11 [2025-04-24 14:07] (current) Merja Herzig |
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| - | In an ordinary radiochemical laboratory the alpha-emitting radionuclides studied are those listed in Table XI.I. Of these Po, Ra, Th and U isotopes are naturally occurring radionuclides while Pu and Am [[textbook: | + | In an ordinary radiochemical laboratory the alpha-emitting radionuclides studied are those listed in [[textbook: |
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| Table XI.I. Most typical alpha-emitting radionuclides studied in radiochemical laboratories. | Table XI.I. Most typical alpha-emitting radionuclides studied in radiochemical laboratories. | ||
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| ===== 11.1.Semiconductor detectors for alpha spectroscopy ===== | ===== 11.1.Semiconductor detectors for alpha spectroscopy ===== | ||
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| - | Semiconductor detectors were discussed already in chapter IX where gamma spectrometry was described. In gamma spectrometry the detector material is germanium whereas in alpha spectrometry the material is silicon. The principal idea in both is the same. They are both diodes composing of an n-type Si/Ge, having an electron donor additive, such as phosphorus, P(V), and a p-type Si/Ge, having an electron acceptor additive, such as boron, B(III). When these are attached to each other a depletion zone is developed around the interface due to combination of electrons and holes on the interface. When a reverse bias voltage is applied across the crystal this depletion zone widens. These are transferred close to electrodes due to the voltage applied. The thickness of the zone is dependent on the voltage applied being typically only 40-60 V in silicon alpha detectors. For the detection of gamma rays the depletion zone needs to be thick, several centimeters, | + | Semiconductor detectors were discussed already in chapter IX where [[textbook: |
| silicon, only 30 μm. Typically the depletion zone in silicon detectors used in alpha spectrometry is 100-200 μm. There are two types of silicon detectors in production (Figure XI.1): surface barrier detectors (SBB) and passivated ion-implanted detectors (PIPS) the latter being a more modern construction mode. | silicon, only 30 μm. Typically the depletion zone in silicon detectors used in alpha spectrometry is 100-200 μm. There are two types of silicon detectors in production (Figure XI.1): surface barrier detectors (SBB) and passivated ion-implanted detectors (PIPS) the latter being a more modern construction mode. | ||
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| - | The detectors are rather small in size (Figure XI.2.) Their diameters are only 2 to 4 centimeters and thickness less than 500 µm (about 1 cm including the metallic cover). Table XI.II. shows properties of alpha detectors available from Canberra. As seen from the table the resolution is very good, 20-40 keV. Resolution is here determined for < | + | The detectors are rather small in size (Figure XI.2.) Their diameters are only 2 to 4 centimeters and thickness less than 500 µm (about 1 cm including the metallic cover). Table XI.II. shows properties of alpha detectors available from Canberra. As seen from the table the [[textbook: |
| - | for the smallest detector in Table XI.II compared to the largest. The background in alpha detectors is very low, only 4-16 counts per day being directly proportional to the surface area of the detector. The background is almost solely caused by cosmic radiation. It is evident that the counting efficiency is better for the larger crystals. Thus one needs to make a compromise with respect to resolution on the one hand and to counting efficiency on the other when selecting a detector. The | + | selection depends naturally on what is needed, high resolution or high efficiency. When measuring |
| - | selection depends naturally on what is needed, high resolution or high efficiency. When measuring alpha activities in environmental and biological samples the activity levels are typically very low requiring very long counting times. In this case a larger detector would be desirable. On the other hand, the background pulses increase with detector size and the overall performance is also dependent on the sample size in comparison with the detector size. In some cases highest possible resolution is a priority. For example, typically < | + | |
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| ===== 11.2. Alpha spectrometry ===== | ===== 11.2. Alpha spectrometry ===== | ||
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| - | Figure XI.3 shows the components of an alpha spectrometer. The planar sample is placed in front of the detector and close to it. Both detector and sample are placed in a vacuum chamber to prevent absorption of alpha particles in air. The voltage (40-60 V) across the detector is supplied by the bias supply. Pulses created in the detector are amplified first in a preamplifier and then in a linear amplifier. The pulses are transformed into digital form in analog-to-digital-converter (ADC) and directed into multichannel | + | Figure XI.3 shows the components of an alpha spectrometer. The planar sample is placed in front of the detector and close to it. Both detector and sample are placed in a vacuum chamber to prevent |
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| Figure XI.3. Electronics in alpha spectrometry (http:// | Figure XI.3. Electronics in alpha spectrometry (http:// | ||
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| - | Prior to counting the alpha-emitting radionuclides, | + | Prior to counting the [[textbook: |
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| ===== 11.3. Sample preparation for alpha spectrometry ===== | ===== 11.3. Sample preparation for alpha spectrometry ===== | ||
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| - | At the end of a radiochemical separation procedure a counting source for alpha spectrometry is prepared. This is done either by electrodeposition of the target element on a steel plate or by microcoprecipitation. The purpose of both methods is to produce a very thin counting source to prevent absorption of alpha radiation in the source. With this respect the electrodeposition method yields a better, thinner, source but in most cases the performance of microcoprecipitation is also satisfactory. In electrodeposition, | + | At the end of a radiochemical separation procedure a counting source for alpha spectrometry is prepared. This is done either by electrodeposition of the target element on a steel plate or by microcoprecipitation. The purpose of both methods is to produce a very thin counting source to prevent |
| - | containing the target nuclide is poured into an electrodeposition vessel and mixed with ammonium, sulphate, chloride, oxalate, hydroxide or formate as electrolyte and the solution is made slightly acidic. A metal disk - usually made of polished steel or sometimes platinum - is tightly mounted to the lower part of the electrodeposition vessel. A platinum wire is put in the vessel and a constant current (10-150 mA/ | + | |
| - | platinum wire operates as anode and the metal disk as cathode (Figure XI.4). The current causes a reduction of the metals in the solution and their deposition in metallic form or as hydroxides on the surface of the steel plate. < | + | |
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| - | The activity of an alpha-emitting radionuclide in an alpha spectrometric measurement following a radiochemical separation is typically determined by adding a tracer to the sample prior to the radiochemical separation. The tracer is another alpha-emitting radionuclide of the target radionuclide element. For all radionuclides given in Table XI.I except < | + | The [[textbook: |
| - | the course of the separation procedure and the same fraction of both isotopes is recovered in the counting source. Due to their different alpha energies the two isotopes can be distinguished from alpha spectrum (Figure XI.5). The initial activity of < | + | the course of the separation procedure and the same fraction of both isotopes is recovered in the counting source. Due to their different alpha energies the two isotopes can be distinguished from alpha spectrum ([[textbook: |
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textbook/nrctextbook/chapter11.1740402746.txt.gz · Last modified: 2025-02-24 14:12 by Merja Herzig