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C.5.2 Beta particle detectors

The following beta particle analysis systems are described:

  • Gas-flow proportional counter;
  • Liquid scintillation spectrometer (LSC);
System: GAS-FLOW PROPORTIONAL COUNTER
Lab/Field: Lab
Radiation detected
Primary Alpha, Beta
Secondary Gamma
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Applicability to site surveys: This system can determine the gross alpha or gross beta activity of water, soil, air filters, or swipes. Results can indicate if nuclide-specific analysis is needed.
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Operation: The system consists of a gas-flow detector, supporting electronics, and an optional guard detector for reducing background count rate. A thin window can be placed between the gas-flow detector and sample to protect the detector from contamination, or the sample can be placed directly into the detector. Systems with guard detectors operate sample and guard detectors in anti-coincidence mode to reduce the background and MDC. The detector high voltage and discriminator are set to count alpha radiation, beta radiation, or both simultaneously. The alpha and beta operating voltages are determined for each system by placing an alpha source, like 230Th or 241Am, in the detector and increasing the high voltage incrementally until the count rate becomes constant, then repeating with a beta source, like 90Sr. The alpha plateau, or region of constant count rate, should have a slope < 2%/100V and be > 800V long. The beta plateau should have a slope of < 2.5%/100V and be > 200V long. Operation on the beta plateau will also allow detection of some gamma radiation and bremsstrahlung (X-rays), but the efficiency is very low. Crosstalk between the α-to-β channels is typically around 10% while β-to-α channels should be < 1%. The activity in soil samples is chemically extracted, separated if necessary, deposited in a thin layer in a planchet to minimize self absorption, and heated to dryness. Liquids are deposited and dried, while air filters and swipes are placed directly in the planchet. After each sample is placed under the detector, P-10 counting gas constantly flows through the detector. Systems with automatic sample changers can analyze tens to hundreds of planchet samples in a single run.
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Specificity/sensitivity: Natural radio-nuclides present in soil samples can interfere with the detection of other contaminants. Unless the nature of the contaminant and any naturally-occurring radio-nuclides is well known, this system is better used for screening samples. Although it is possible to use a proportional counter to roughly determine the energies of alpha and beta radiation, the normal mode of operation is to detect all alpha events or all alpha and beta events. Some systems use a discriminator to separate alpha and beta events, allowing simultaneous determination of both the alpha and beta activity in a sample. These systems do not identify the alpha or beta energies detected and cannot be used to identify specific radio-nuclides. The alpha channel background is very low, < 0.2 cpm (< 0.04 cpm guarded), depending on detector size. Typical, 4-pi, efficiencies for very thin alpha sources are 35-45% (window) and 40-50% (windowless). Efficiency depends on window thickness, particle energy, source-detector geometry, backscatter from the sample and holder, and detector size. The beta channel background ranges from 2 to 15 cpm (< 0.5 cpm guarded). The 4-pi efficiency for a thin 90Sr/90Y source is >50% (window) to >60% (windowless), but can reduce to <5% for a thick source. MDA’s for guarded gas-flow proportional counters are somewhat lower for beta emitters than for internal proportional counters because of the lower backgrounds. Analyzing a high radioactivity sample or flushing the detector with P-10 gas at too high a flow rate can suspend fine particles and contaminate the detector.
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Cost of equipment $4,000-$5,000 (manual), $25,000-$30,000K (automatic) (year 2002).
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Cost per measurement $30 to $50 plus radiochemistry (year 2002).

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System: LIQUID SCINTILLATION SPECTROMETER (LSC)
Lab/Field: Lab (primarily), field (secondarily)
Radiation detected
Primary Alpha, beta
Secondary Gamma
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Applicability to site surveys: Liquid Scintillation can be a very effective tool for measuring the concentration of radio-nuclides in soil, water, air filters, and swipes. Liquid scintillation has historically been applied more to beta emitters, particularly the low energy beta emitters 3H and 14C, but it can also apply to other radio-nuclides. More recently it has been used for measuring radon in air and water. Initial scoping surveys may be done (particularly for loose surface contamination) with surface swipes or air particulate filters. They may be counted directly in liquid scintillation cocktails with no paper dissolution or other sample preparation.
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Operation: The liquid scintillation process involves detection of light pulses (usually in the near visible range) by photo-multiplier tubes (or conceptually similar devices). The detected light pulses originate from the re-structuring of previously excited molecular electron structures. The molecular species that first absorb and then re-admit the visible light are called ‘liquid scintillators’ and the solutions in which they reside are called ‘liquid scintillation cocktails’. For gross counting, samples may be placed directly into a LSC vial of cocktail, and counted with no preparation. Inaccuracies result when the sample itself absorbs the radiation before it can reach the LSC cocktail, or when the sample absorbs the light produced by the cocktail. For accurate results, these interferences are minimized. Interferences in liquid scintillation counting due to the inability of the solution to deliver the full energy pulse to the photo-multiplier detector, for a variety of reasons, are called ‘pulse quenching’. Raw samples that cloud or colour the LSC cocktail so that the resulting scintillations are absorbed will ‘quench’ the sample and result in underestimates of the activity. Such samples are first processed by ashing, radiochemical or solvent extraction, or pulverizing to place the sample in intimate contact with the LSC cocktail. Actions like bleaching the sample may also be necessary to make the cocktail solution transparent to the wavelength of light it emits. The analyst has several reliable computational or experimental procedures to account for ‘quenching’. One is by exposing the sample and pure cocktail to an external radioactive standard and measuring the difference in response.
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Specificity/sensitivity: The method is extremely flexible and accurate when used with proper calibration and compensation for quenching effects. Energy spectra are 10 to 100 times broader than gamma spectrum photo-peaks so that quantitative determination of complex multi-energy beta spectra is impossible. Sample preparation can range from none to complex chemical reactions. In some cases, liquid scintillation offers many unique advantages such as no sample preparation before counting in contrast to conventional sample preparation for gas proportional counting. Recent advances in electronic stability and energy pulse shape discrimination has greatly expanded uses. Liquid scintillation counters are ideal instruments for moderate to high energy beta as well as alpha emitters, where the use of pulse shape discrimination has allowed dramatic increases in sensitivity by electronic discrimination against beta and gamma emitters. Additionally, very high energy beta emitters (above 1.5 MeV) may be counted using liquid scintillation equipment without ‘liquid scintillation cocktails’ by use of the Cerenkov light pulse emitted as high energy charged particles move through water or similar substances.
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Cost of equipment $20,000 to $70,000 based on the specific features and degree of automation (year 2002).
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Cost per measurement $50 -200 plus cost of chemical separation, if required (year 2002).