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C.4.5 X-ray and low energy gamma detectors

The following X-ray and low energy gamma detectors are described:

  • Fidler probe with survey meter;
  • Field X-ray fluorescence spectrometer;

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System: FIDLER PROBE WITH SURVEY METER
Field: Field
Radiation detected
Primary X-ray
Secondary Low Energy Gamma
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Applicability to site surveys: The FIDLER (Field Instrument for the Detection of Low Energy Radiation) probe is a specialized detector consisting of a thin layer of sodium or caesium iodide which is optimized to detect gamma and X-radiation below 100 keV. It is most widely used for determining the presence of Pu and 241Am, and can be used for estimating radio-nuclide concentrations in the field.
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Operation: The FIDLER consists of a thin beryllium or aluminium window, a thin crystal of sodium iodide, a quartz light pipe, and photomultiplier tube. The probe can have either a 3 in. or 5 in. crystal. The discussion below is applicable to 5 in. crystals. The survey meter requires electronics capable of setting a window about an X-ray or gamma ray energy. This window allows the probe and meter to detect specific energies and, in most cases, provide information about a single element or radionuclide. The window also lowers the background count. Two types of survey meters are generally used with FIDLER probes. One type resembles those used with GM and alpha scintillation probes. They have an analog meter and range switch. The second type is a digital survey meter, which can display the count rate or accumulate counts in a scaler mode for a preset length of time. Both types have adjustable high voltage and window settings. The advantage of the digital meter is that both background and sample counts can be acquired in scaler mode, yielding a net count above background. The activity of a radionuclide can then be estimated in the field.
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Specificity/sensitivity: The FIDLER probe is quite sensitive to X-ray and low energy gamma radiation. Since it has the ability to discriminate energies, an energy window can be set that makes it possible to determine the presence of specific radio-nuclides when the nature of the contamination is known. If the identity of a contaminant is known, the FIDLER can be used to quantitatively determine the concentration. However, interferences can cause erroneous results if other radio-nuclides are present. The FIDLER can also be used as a survey instrument to detect the presence of X-ray or low energy gamma contaminates, and to determine the extent of the contamination. FIDLER probes are most useful for determining the presence of Pu and 241Am.
These isotopes have a complex of X-rays and gamma rays from 13-21 keV that have energies centred around 17 keV, and 241Am has a gamma at 59 keV. There is an interference at 13 keV from both americium and uranium X-rays. The FIDLER cannot distinguish which isotope of Pu is present. 241Am can be identified based on the 59 keV gamma. Typical sensitivities for 238Pu and 239Pu at one foot above the surface of a contaminated area are 500 to 700 and 250 to 350 counts per minute per μCi per square meter (cpm/μCi/m2), respectively. Assuming a soil density of 1.5, uniform contamination of the first 1 mm of soil, and a typical background of 400 counts per minute, the MDC for 238Pu and 239Pu would be 370 and 740 Bq/kg (10 and 20 pCi/g), or 1500 and 3000 Bq/m2 (900 and 1,800 dpm/100 cm2). This MDC is for fresh deposition; and will be significantly less as the plutonium migrates into the soil. Because the window is fragile, most operations with a FIDLER probe require a low mass protective cover to prevent damaging the window. Styrofoam, cardboard, and other cushioning materials are common choices for a protective cover.
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Cost of equipment $4,000 to $7,000 (year 2002).
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Cost per measurement $10 to $20 (year 2002).

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System: FIELD X-RAY FLUORESCENCE SPECTROMETER
Field: Field
Radiation detected
Primary X-ray and low energy gamma radiation
Secondary None
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Applicability to site surveys: The system accurately measures relative concentrations of metal atoms in soil or water samples down to the ppm range.
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Operation: This system is a rugged form of X-ray fluorescence system that measures the characteristic X-rays of metals as they are released from excited electron structures. The associated electronic and multi-channel analyzer systems are essentially identical to those used with germanium spectrometry systems. The spectra of characteristic X-rays give information for both quantitative and qualitative analysis; however, most frequently, the systems are only calibrated for relative atomic abundance or percent composition.
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Specificity/sensitivity: This is ideal for cases of contamination by metals that have strong X-ray emissions within 5-100 keV. Application for quantification of the transition metals (in the periodic table) is most common because of the X-ray emissions. Operation of this equipment is possible with only a moderate amount of training. The sensitivity ranges from a few percent to ppm depending on the particular atoms and their characteristic X-rays. When converted to activity concentration, the minimum detectable concentration for 238U is around 1,850 Bq/kg (50 pCi/g) for typical soil matrices.
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Cost of equipment $15,000 – $75,000 depending on size, speed of operation and auxiliary features employed for automatic analysis of the results (year 2002).
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Cost per measurement $200 (year 2002).