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C.4.1 Alpha particle detectors

The following alpha particle detectors are described:

  • Alpha scintillation survey meter;
  • Alpha track detector;
  • Electret ion chamber;
  • Gas-flow proportional counter;
  • Long range alpha detector (LRAD).
System: ALPHA SCINTILLATION SURVEY METER
Lab/Field: Field
Radiation detected
Primary Alpha
Secondary None (in relatively low gamma fields)
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Applicability to site surveys: The alpha scintillation survey meter is useful for determining the presence or absence of alpha-emitting contamination on nonporous surfaces, swipes, and air filters, or on irregular surfaces if the degree of surface shielding is known.
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Operation: This survey meter uses an alpha radiation detector with a sensitive area of approximately 50 to 100 cm2 (8 to 16 in.2). The detector has a thin, aluminized window of mylar that blocks ambient light but allows alpha radiation to pass through. The detecting medium is silver activated zinc sulfide, ZnS(Ag). When the discriminator is appropriately adjusted, the meter is sensitive only to alpha radiation. Light pulses are amplified by a photomultiplier tube and passed to the survey meter.
The probe is generally placed close to the surface due to the short range of alpha particles in air. A scanning survey is used to identify areas of elevated surface contamination and then a direct survey is performed to obtain actual measurements. Integrating the readings over time improves the sensitivity enough to make the instrument very useful for alpha surface contamination measurements for many isotopes. The readings are displayed in counts per minute, but factors can usually be obtained to convert readings from cpm to dpm. Conversion factors, however, can be adversely affected by the short range of alpha particles which allows them to be shielded to often uncertain degrees if they are embedded in the surface. Systems typically use 2 to 6 ‘C’ or ‘D’ cells and will operate for 100-300 hours.
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Specificity/sensitivity: When the alpha discriminator is correctly adjusted, the alpha scintillation survey meter measures only alpha radiation, even if there are other radiations present. A scanning survey gives a quick indication of the presence or absence of surface contamination, while integrating the readings provides a measure of the activity on a surface, swipe, or filter. Alpha radiation is easily adsorbed by irregular, porous, moist, or over painted surfaces, and this should be carefully considered when converting count rate data to surface contamination levels. This also requires wet swipes and filters to be dried before counting. The minimum sensitivity is around 10 cpm using the needle deflection or 1 to 2 cpm when using headphones or a scaler. Some headphones or scalers give one click for every two counts, so the manual should be consulted to preclude underestimating the radioactivity by a factor of two.
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Cost of equipment $1,000 (year 2002)
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Cost per measurement $5 (year 2002)

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System: ALPHA TRACK DETECTOR
Lab/Field: Field and indoor surfaces
Radiation detected
Primary Alpha
Secondary None
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Applicability to site surveys: Alpha track detectors measure gross alpha surface contamination, soil activity levels, or the depth profile of contamination.
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Operation: This is a passive integrating detector. It consists of a l mm-thick sheet of polycarbonate material which is deployed directly on the soil surface or in close proximity to the contaminated surface. When alpha particles strike the detector surface, they cause microscopic damage centres to form in the plastic matrix. After deployment, the detector is etched in a caustic solution which preferentially attacks the damage centres. The etch pits may then be counted in an optical scanner. The density of etch pits, divided by the deployment time, is proportional to the soil or surface alpha activity. The measurement may be converted to isotopic concentration if the isotopes are known or measured separately. The area of a standard detector is 2 cm2 (0.3 in.2), but it may be cut into a variety of shapes and sizes to suit particular needs.
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Specificity/sensitivity: Alpha track detectors are relatively inexpensive, simple, passive, and have no measurable response to beta/gamma radiation. They provide a gross alpha measurement where the lower limit of detection is a function of deployment time. For surface contamination it is 330 Bq/m2 (200 dpm/100 cm2) @ l hour, 50 Bq/m2 (30 dpm/100 cm2) @ 8 hours, and 17 Bq/m2 (10 dpm/100 cm2) @ 48 hours. For soil contamination it is 11,000 Bq/kg (300 pCi/g) @ 1 hour, 3,700 Bq/kg (100 pCi/g) @ 8 hours, and 740 Bq/kg (20 pCi/g) @ 96 hours. High surface contamination or soil activity levels may be measured with deployment times of a few minutes, while activity down to background levels may require deployment times of 48-96 hours. When placed on a surface, they provide an estimate of alpha surface contamination or soil concentration. When deployed against the side of a trench, they can provide an estimate of the depth profile of contamination. They may also be used in pipes and under/inside of equipment.
For most applications, the devices are purchased for a fixed price per measurement, which includes readout. This requires that the detectors be returned to the vendor and the data are not immediately available. For programs having continuing needs and a large number of measurements, automated optical scanners may be purchased. The cost per measurement is then a function of the number of measurements required.
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Cost of equipment $65,000 (year 2002).
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Cost per measurement $5 to $10 (year 2002).

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System: ELECTRET ION CHAMBER
Lab/Field: Field
Radiation detected
Primary Alpha, beta, gamma, or radon
Secondary None
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Applicability to site surveys: An electret is a passive integrating detector for measurements of alpha- or beta-emitting contaminants on surfaces and in soils, gamma radiation dose, or radon air concentration.
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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 (l5 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 also serves as a screen to determine whether or not more nuclide-specific analyses may be 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: LONG RANGE ALPHA DETECTOR (LRAD)
Lab/Field: Field
Radiation detected
Primary Alpha
Secondary None
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Applicability to site surveys: The LRAD is a rugged field-type unit for measuring alpha surface soil concentration over a variety of dry, solid, flat terrains.
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Operation: The LRAD system consists of a large (1 m x 1 m) aluminium box, open on the bottom side, containing copper plates that collect ions produced in the soil or surface under the box, and used to measure alpha surface contamination or soil concentration. It is attached to a lifting device on the front of a tractor and can be readily moved to new locations. Bias power is supplied by a 300-V dry cell battery, and the electrometer and computer are powered by an automobile battery and DC-to-AC inverter. A 50 cm grounding rod provides electrical grounding. A notebook computer is used for data logging and graphical interpretation of the data. Alpha particles emitted by radio-nuclides in soil travel only about 3 cm in air. However, these alpha particles interact with the air and produce ions that travel considerably farther. The LRAD detector box is lowered to the ground to form an enclosed ionization region. The copper detector plate is raised to +300V along with a guard detector mounted above the detector plate to control leakage current. The ions are then allowed to collect on the copper plate producing a current that is measured with a sensitive electrometer. The signal is then averaged and processed on a computer. The electric current produced is proportional to the ionization within the sensitive area of the detector and to the amount of alpha contamination present on the surface soil.
Due to its size and weight (300 lb), the unit can be mounted on a tractor for ease of movement. All metal surfaces are covered with plastic to reduce the contribution from ion sources outside the detector box. At each site, a ground rod is driven into the ground. Each location is monitored for at least 5 min. After each location is monitored, its data is fed into a notebook computer and an interpolative graph of alpha concentration produced. The unit is calibrated using standard alpha sources.
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Specificity/sensitivity: The terrain over which this system is used must be dry, to prevent the shielding of alpha particles by residual moisture, and flat, to prevent air infiltration from outside the detector, both of which can lead to large errors. The unit can detect a thin layer of alpha surface contamination at levels of 33-83 Bq/m2 (20-50 dpm/100 cm2), but does not measure alpha contamination of deeper layers. Alpha concentration errors are 74-740 Bq/kg (2-20 pCi/g), with daily repeat accuracies of 370-3,700 Bq/kg (10-100 pCi/g), depending on the contamination level. The dynamic measurement range appears to be 370-110,00 Bq/kg (10-3,000 pCi/g).
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Cost of equipment $25,000 (estimate for tractor, computer, software, electrometer, and detector) (year 2002).
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Cost per measurement $80 (based on 30 min per point and a 2 person team) (year 2002).