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C.4.2 Beta particle detectors

The following beta particle detectors are described:

  • Electret ion chamber;
  • Gas-flow proportional counter;
  • GM survey meter with beta pancake probe.
System: ELECTRET ION CHAMBER
Lab/Field: Field
Radiation detected
Primary Low energy beta (e.g., tritium, 99Tc, 14C, 90Sr, 63Ni), alpha, gamma, or radon
Secondary None
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Applicability to site surveys: Applicability to site surveys: This system measures alpha- or beta-emitting contaminants on surfaces and in soils, gamma radiation dose, or radon air concentration, depending on how it is configured.
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Operation: The system consists of a charged teflon disk (electret), open-faced ionization chamber, and electret voltage reader/data logger. When the electret is screwed into the chamber, a static electric field is established and a passive ionization chamber is formed. For alpha or beta radiation, the chamber is opened and deployed directly on the surface or soil to be measured so the particles can enter the chamber. For gammas, however, the chamber is left closed and the gamma rays incidenting on the chamber penetrate the 2 mm-thick plastic detector wall. These particles or rays ionize the air molecules, the ions are attracted to the charged electret, and the electret’s charge is reduced. The electret charge is measured before and after deployment with the voltmeter, and the rate of change of the charge is proportional to the alpha or beta surface or soil activity, with appropriate compensation for background gamma levels. A thin mylar window may be used to protect the electret from dust. In low-level gamma measurements, the electret is sealed inside a mylar bag during deployment to minimize radon interference. For alpha and beta measurements, corrections must be made for background gamma radiation and radon response. This correction is accomplished by deploying additional gamma or radon-sensitive detectors in parallel with the alpha or beta detector. Electrets are simple and can usually be reused several times before recharging by a vendor. Due to their small size (3.8 cm tall by 7.6 cm diameter or l.5 in. tall by 3 in. diameter), they may be deployed in hard-to-access locations.
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Specificity/sensitivity: This method gives a gross alpha, gross beta, gross gamma, or gross radon measurement. The lower limit of detection depends on the exposure time and the volume of the chamber used. High surface alpha or beta contamination levels or high gamma radiation levels may be measured with deployment times of a few minutes. Much lower levels can be measured by extending the deployment time to 24 hours or longer. For gamma radiation, the response of the detector is nearly independent of energy from 15 to 1200 keV, and fading corrections are not required. To quantify ambient gamma radiation fields of 10 μR/hr, a 1000 ml chamber may be deployed for two days or a 50 ml chamber deployed for 30 days. The smallest chamber is particularly useful for long term monitoring and reporting of monthly or quarterly measurements. For alpha and beta particles, the measurement may be converted to isotopic concentration if the isotopes are known or measured separately. The lower limit of detection for alpha radiation is 83 Bq/m2 (50 dpm/100 cm2) @ 1 hour, 25 Bq/m2 (15 dpm/100 cm2) @ 8 hours, and 13 Bq/m2 (8 dpm/100 cm2) @ 24 hours. For beta radiation from tritium it is 10,000 Bq/m2 (6,000 dpm/cm2) @ 1 hour and 500 Bq/m2 (300 dpm/cm2) @ 24 hours. For beta radiation from 99Tc it is 830 Bq/m2(500 dpm/cm2) @ 1 hour and 33 Bq/m2 (20 dpm/cm2) @ 24 hours.
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Cost of equipment $4,000 to $25,000, for system if purchased (year 2002).
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Cost per measurement $8-$25, for use under service contract (year 2002).

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System: GAS-FLOW PROPORTIONAL COUNTER
Lab/Field: Field
Radiation detected
Primary Alpha, beta
Secondary Gamma
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Applicability to site surveys: This equipment measures gross alpha or gross beta/gamma surface contamination levels on relatively flat surfaces like the floors and walls of facilities. It would serve as a screen to determine whether or not more nuclide-specific analyses were needed.
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Operation: This system consists of a gas-flow proportional detector, gas supply, supporting electronics, and a scaler or rate meter. Small detectors (~100 cm2) are hand-held and large detectors (~400-600 cm2) are mounted on a rolling cart. The detector entrance window can be < 1 to almost 10 mg/cm2 depending on whether alpha, alpha-beta, or gamma radiation is monitored. The gas used is normally P-10, a mixture of 10% methane and 90% argon. The detector is positioned as close as practical to the surface being monitored for good counting efficiency without risking damage from the detector touching the surface. Quick disconnect fittings allow the system to be disconnected from the gas bottle for hours with little loss of counting efficiency. The detector operating voltage can be set to make it sensitive only to alpha radiation, to both alpha and beta radiation, or to beta and low energy gamma radiation. These voltages are determined for each system by placing either an alpha source, such as 230Th or 241Am, or a beta source, such as 90Sr, facing and near the detector window, then increasing the high voltage in incremental steps until the count rate becomes constant. The alpha plateau, the region of constant count rate, will be almost flat. The beta plateau will have a slope of 5 to 15 percent per 100 volts. Operation on the beta plateau allows detection of some gamma radiation, but the efficiency is very low. Some systems use a spectrometer to separate alpha, and beta/gamma events, allowing simultaneous determination of both the alpha and beta/gamma surface contamination levels.
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Specificity/sensitivity: These systems do not identify the alpha or beta energies detected and cannot be used to identify specific radio-nuclides. Background for operation on the alpha plateau is very low, 2 to 3 counts per minute, which is higher than for laboratory detectors because of the larger detector size. Background for operation on the beta plateau is dependent on the ambient gamma and cosmic ray background, and typically ranges from several hundred to a thousand counts per minute. Typical efficiencies for un-attenuated alpha sources are 15-20%. Beta efficiency depends on the window thickness and the beta energy. For 90Sr/90Y in equilibrium, efficiencies range from 5% for highly attenuated to about 35% for un-attenuated sources. Typical gamma ray efficiency is < 1%. The presence of natural radio-nuclides in the surfaces could 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 assessing gross surface contamination levels. The texture and porosity of the surface can hide or shield radioactive material from the detector, causing levels to be underestimated. Changes in temperature can affect the detector sensitivity. Incomplete flushing with gas can cause a non-uniform response over the detector surface. Condensation in the gas lines or using the quick disconnect fittings can cause count rate instability.
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Cost of equipment $2,000 to $4,000 (year 2002).
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Cost per measurement $2-$10 per m2 (year 2002).

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System: GM SURVEY METER WITH BETA PANCAKE PROBE
Lab/Field: Field
Radiation detected
Primary Beta
Secondary Gamma and alpha
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Applicability to site surveys: This instrument is used to find and measure low levels of beta/gamma contamination on relatively flat surfaces.
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Operation: This instrument consists of a flat ‘pancake’ type Geiger-Mueller detector connected to a survey meter which measures radiation response in counts per minute. The detector housing is typically a rigid metal on all sides except the radiation entrance face or window, which is made of mylar, mica, or a similar material. A steel, aluminium, lead, or tungsten housing surrounds the detector on all sides except the window, giving the detector a directional response. The detector requires approximately 900 volts for operation. It is held within a few cm of the surface to minimize the thickness of air shielding in between the radioactive material and the detector. It is moved slowly to scan the surface in search of elevated readings, then held in place long enough to obtain a stable measurement. Radiation entering the detector ionizes the gas, causes a discharge throughout the entire tube, and results in a single count being sent to the meter. The counts per minute meter reading is converted to a beta surface contamination level in the range of 1,700 Bq/m2 (1,000 dpm/100 cm2) using isotope specific factors.
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Specificity/sensitivity: Pancake type GM detectors primarily measure beta count rate in close contact with surfaces to indicate the presence of contamination. They are sensitive to any alpha, beta, or gamma radiation that enters the detector and causes ionization. As a result, they cannot determine the type or energy of that radiation, except by using a set of absorbers. To be detected, beta particles must have enough energy to penetrate through any surface material that the contamination is absorbed in, plus the detector window, and the layer of air and other shielding materials in between. Low energy beta particles from emitters like 3H (17 keV) that cannot penetrate the window alone are not detectable, while higher energy betas like those from 60Co (314 keV) can be readily detected. The beta detection efficiency at a field site is primarily a function of the beta energy, window thickness, and the surface condition. The detection sensitivity can be improved by using headphones or the audible response during scans. By integrating the count rate over a longer period or by counting the removable radioactive material collected on a swipe, the ability to detect surface contamination can be improved. The nominal 2 in. diameter detector can measure an increase of around 100 cpm above background, which equates to 4,200 Bq/m2 (2,500 dpm/100 cm2) of 60Co on a surface under the detector or 20 Bq (500 pCi) on a swipe. Larger 100 cm2 detectors improve sensitivity and eliminate the need to swipe. A swipe’s collection efficiency may be below 100%, and depends on the wiping technique, the actual surface area covered, the texture and porosity of the surface, the affinity of the contamination for the swipe material, and the dryness of the swipe. This will proportionately change the values above. The sensitivity to gamma radiation is around 10% or less of the beta sensitivity, while the alpha detection efficiency is difficult to evaluate.
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Cost of equipment $400 to $1,500 (year 2002).
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Cost per measurement $5 to $10 per location (year 2002).