Nal/TL scintillation detector

Figure 9.3 The whole-body counter of University Medical Centre, Utrecht, The Netherlands. The counter has a mobile shadow shield with two Nal(Tl) scintillation detectors (4x6 in) placed at opposite sites of the subject. The lead shielding is 100 cm long with a diameter of 90 cm. The scanner moves on rails over a distance of 240 cm with an adjustable speed. Extreme variations in geometry yield practically the same value for 59Fe activity. The equipment can be used for measurements in man and small animals.

For the measurement of y emitters in solids Nal(Tl) scintillation detectors or Ge detectors are most suitable, depending upon whether high counting efficiency or high energy resolution is required. For comparison, the spectra of Co taken with a Nal(Tl) scintillation detector and with a Ge(Li) detector are plotted in Fig. 7.16. 

Figure 7.16. y-ray spectra of Co taken with a Nal (Tl) scintillation detector and a Ge (Li) semiconductor detector. 

These detectors are made of semiconducting materials. In these detectors, solid-state electrodes are made from Li doped with Si or Ge. The resolution is approximately 1-2 keV for 1 MeV y-Rays and sometimes provides a greater than 10-fold improvement over Nal (Tl) scintillation detectors, described below. These are commercially available and more often used in research-grade instruments. 

A JASCO PU 1580 high performance liquid chromatography (HPLC) system equipped with a PU 1575 UV/VIS detector (JASCO, Japan) was used in the studies. A well type Nal(Tl) scintillation detector was coupled to the system to measure activity in the eluate. All solvents used for HPLC analysis were of HPLC grade, purchased from reputed local manufacturers, and degassed and filtered prior to use. 

For measuring Tc and other gamma emitters, a Nal(Tl) scintillation detector is used. The resolution of the scanner is dependent on the width of the slit-collimator, the distance between chromatogram and detector, and the window settings on the scaler. Artificial results may be obtained if the peaks are not symmetrical and comparable. 

The passage of each radioactive particle was registered by nine y detectors (Nal/Tl scintillation detectors), encased in lead shields and placed along the... 

Figure 8.25 The Nal(Tl) scintillation detector, (a) The assembled detector (b) a schematic representation of the photomultiplier and its circuitry. [Courtesy of ORTEC (Ametek) (www.ortec-online.com). From Jenkins et al., 1981, used with permission.]...

The Nal(Tl) scintillation detector is most useful for short-wavelength X-rays, <2 A (Z > 27), so it complements the proportional counter. It also has the potential for escape peaks caused by the iodine K line (about 30 keV or 0.374 A). Incoming X-rays with wavelengths less than 0.374 A will result in escape peaks about 30 keV lower in energy than the true energy. The major disadvantage of the Nal(Tl) detector is that its resolution is much worse than that of the proportional counter. This is due to the wider pulse height distribution that results in the output pulse because of the multiple steps involved in the operation of this detector. 

If a semiconductor (such as hyper-pure germanium) is used instead of an Nal(Tl) scintillation detector, the resolution is improved by a factor of 30 and more. The tremendous increase in resolution compared to scintillation detectors is shown in Figure 5. Here, a spectrum is shown of lead bullet... 

For a similar Ge lithium-drifted detector, the energy required for ionization is 2.96 eV. This is much less than the energy required for ionization in a proportional counter or a Nal(Tl) scintillation detector. 

Figure 2. Comparison of gamma spectrum of (gamma energy 661.6 keV) obtained with 7,5x7.5-cm Nal(Tl) scintillation detector (a) and with a 50-cm Ge(Li) semiconductor detector (b)...

Germanium detectors are characterized by three parameters resolution, peak-to-Compton ratio, and efficiency. The resolution is typically given for the 1332-keV Co line and varies from 1.8 keV for the very best to 2.3 keV for the very large detectors. The peak-to-Compton ratio is measured as the ratio of the number of counts in the 1332-keV peak to the number of counts in a region of the Compton continuum. Values vary from 30 to 90 for the most expensive model. The efficiency is expressed as a relative efficiency compared with the 7.5x7.5-cm Nal(Tl) scintillation detector. Relative efficiencies of HP-Ge detectors vary from 10% up to 150%. The dead time of semiconductor detectors is low, so the count rate is limited largely by the electronic circuit. 

Counting of /3 activity is seldom required because all of the rare earths except Y emit y-rays which are more conveniently measured. In early work, Nal(Tl) scintillation detectors were used to measure y-ray activity from rare earths which were separated from each other usually with ion exchange columns. Mosen et al. (1%1) measured all sixteen rare earths in meteorites by this technique only Y, Er, and Tm required /3 counting. 

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