Well detectors

Canberra has reported measurements where the efficiency of a well detector is five times better than an 80% coaxial detector. However, the capacitance of a well detector will be higher than for an equivalent coaxial detector, resulting in worse resolution - perhaps 2.3 keV FWHM rather than 2.0 keV. As we saw above in Section 13.2.2, worse resolution means worse (i.e. higher) MDA, meaning that the improvement in MDA expected based on efficiency ratio would not quite be achieved. 

To summarize, well detectors offer significant advantages in terms of counting efficiency, but these advantages are offset by the limitation on sample size and much-increased TCS. Before contemplating buying a well detector, the analyst would be well advised to 

A significant practical difficnlty is that these dimensions do not necessarily accord with the actnal dimensions of detector endcaps. As larger detectors have become available, the diameter of the endcap has had to be increased. Standard endcap sizes are listed in Table 13.5. From that, it is evident that detector sizes of more than 40% cannot accommodate the smaller, 450 ml, lEC 

Snch considerations mean that a number of non-IEC standard Marinelli beakers are in nse. If the analyst were to commission the manufacture of a beaker tailored to 

How much sample can be conveniently prepared for counting This may depend upon economics as much as laboratory resources. A sample may need to be dried, ashed and ground to a small particle size. Can such extra labour intensive activity be justified In a commercial situation, would the client be prepared to pay for it What limit of detection is being aimed for This will be related to sample mass and count period, as we know from Equation (13.1). 

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The HVI plates were leached with 0.1 M HC1. Be-7 was measured using intrinsic germanium coaxial and well detectors. Lead-210 was determined 30 days after collection stopped by separating and measuring Bi-210 (Poet et al., 1972). When Pb-210 was measured, the upper two HVI stages were coated with a thin layer of petroleum jelly to minimize soil particle bounce. 

Assume we count this water solution using a Nal well detector (see Chapter 18) that has an efficiency of 20% for the 0.365-MeV photons from 131I. Assuming one wants to collect 103 counts in a 10-min count (and 81% of the 131I decays result in a 0.365-MeV photon), one estimates a radioactivity in the solution as... 

The exact amount of 1311 in the original precipitate must be known, but the activity of this precipitate is so high as to preclude its direct measurement in the well detector. Therefore, what one does is to prepare a standard dilution of the original I- solution that can be measured in the well detector. 

A Canberra well-detector system using Fitzpeaks gamma spectrometry software was used to determine caesium-137 activities in all sample fractions. Variations in sample height arising from small sample masses were corrected for where appropriate. The detector efficiency calibration was determined using an NPL mixed gamma standard spiked into a sand matrix (Croudace 1991). 

Crystal Scintillation Detector. The well detector is a common type of a crystal scintillation detector and has a hole drilled in the end or side of the cylindrical crystal to accept a test tube. Because it is hygroscopic, the crystal is hermetically sealed in an aluminum can with a transparent quartz window at one end through which the blue-violet (420 nm) scintillations are detected. The photons of gamma emitters, such as Cr, Co, Fe, and (Table 1-14) in the sample easily penetrate the specimen tube and the thin, low-density can and enter the crystal where they are likely to... 

The crystal itself is usually a circular cyHiider machined from a single crystal of sodium iodide that was carefully grown to ensure optical clarity for efficient light collection. A small amount of thallium is added to the mother liquor to improve performance. The high atomic number of iodine and the density of sodium iodide (3.7g/cm ) favor the absorption of y-radiation. For this reason, a well counter is often referred to as a y-counter. For a typical well detector, the counting efficiency for expressed as the percentage of decays that produces counts, approximates 70%. 

The gamma ray scintillation spectrometer (Fig. 1) consisted of two single channel analyzers coupled to a common sodium iodide well detector, preamplifier, amplifier and scalers. By setting each analyzer for the appropriate energy the two isotopes were determined. 

The procedures for bombardment and analysis that we discuss below are described in greater detail in (I). After both the proton and He bombardments, samples were counted in calibrated Nal well detectors to establish absolute source intensity and time-decay characteristics. Sources were simultaneously analyzed by HPLC (with a Nal coincidence detector) and by GC (with a proportional counter detector) to determine their chemical composition. After chemical purification, the sources were again analyzed to determine purity. All absolute activities discussed below are normalized to a zero time at the end of the bombardment. 

The most commonly used isotopic labels for such assays are 1-125, Co-57, H-3, and C-14. However, since 1-125 and Co-57 are gamma emitters, while H-3 and C-14 are beta emitters, two basically different types of nuclear counting equipment are required to detect both types of radiation. These gamma emitters are most conveniently counted with a sodium iodide well detector, while beta emitters must be detected in a liquid scintillation counter. 

Dilution Tube Gamma Well Detector LSC Coincidence Mode Ratio B A... 

The abundance of LEU products is determined with a total uncertainty of 0.13% (Is) by measuring the 186 keV gamma rays with a Nal or GeLi well detector on 5 ml solutions containing a well-known amount of uranium, close to 0.5 g. The volume of the sample and synthetic calibration test solutions, their acid and U concentrations are kept in a very narrow range so that the self-absorption and coimting geometry be constant factors or minor variations be accurately... 

Yokoyama and Nguyen (1980) described a method for the determination of Pb, Th and Ra on filters without acid digestion. The filters are simply dried, folded and placed on the detector in a reproducible geometry. For highest sensitivity measurements, a germanium crystal with a well is recommended. Moore and Dymond (1988,1991) used a germanium well detector to measure Pb and Ra in samples from deep-sea sediment traps. 

However, if the count period were long and the detection efficiency high (perhaps a low-energy gamma-ray emitted with high probability and measured close to the detector or inside a well detector), then the assumptions underpinning our use of the Poisson distribution are no longer valid. It is then necessary to return to the binomial distribution. There is no place here for the mathematics involved but it can be shown that if we observe a count of n, then the expected true count is ... 

It gets worse the larger the detector and is worst of all when using a well detector. 

The standard shape for a scintillation detector is a simple cylinder with its height equal to its diameter. The ease with which such detectors can be made with precisely reproducible dimensions and properties made the concept of a standard detector a reality. For most routine purposes, the detector of choice is the Nal(TI). Typical off-the-shelf sizes are (in inches) 1 x 1,2 x 2 and 3x3, often still quoted in archaic units. End well detectors of similar sizes are also easily available. 

Such a specification would refer to a standard coaxial detector. Specification sheets for other types of detector will contain other infomiation and not all the parameters noted in Table 11.3 will be warranted. For example, for a well detector geometric details of the well and the active volume will be quoted. It is likely that only the detector resolution will be warranted. Peak-to-Compton ratio may be measured, but not warranted. For low-energy detectors, only the resolution wiU be warranted. 

It is not standard practice to warrant relative efficiency for well detectors, but if it were the procedure described about could be used unaltered. The ANSI/IEEE standard includes a procedure for determining the absolute efficiency of well detectors but this has not been adopted generally by the manufacturers and unless specially requested, it would not normally be on the specification sheet. Normally active volume of germanium is quoted as a... 

Figure 13.10 Cross-section of a typical well detector, together with the efficiency at 1332keV from a point source as a function of distance from the bottom of the well (adapted from figures originally provided by Canberra Nuclear)...

Small samples will be counted very efficiently in a well detector, but cascading gammas have a high probability of summing. The sample size constrains the MDA. 

PGT Inc., Well-Detectors with the Through-Hole Advantage http //www.pgt. com/Nuclear/Well. html. 

WELL DETECTOR A detector in which there is a well extending into its body to accommodate the sample. Such detectors have high detection efficiency. 

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