|Fig.1. A schematic sectional view of the SICH-9.1M whole body counter: 1 - a detector's binding joist; 2 - shielding, 200-mm cast iron and lining (lead, 5 mm; cadmium, 1 mm; and copper, 2 mm); 3 - movable bed; 4 - brackets for mounting the rails; 5 - door; 6 - electric motor; and 7 - new phoswich detectors (all measurements in mm).|
An original whole body counter (WBC) named SICH-9.1 has been used since 1974 to measure 90Sr, 137Cs and 40K in people. This unique measurement system was designed by a group of scientists headed by Prof. Yu. Belle specifically for the exposure situation on the Techa River (Belle et al., 1975). Quantification of 90Sr is achieved by measuring with a 'phoswich' detector the bremsstrahlung of 90Y (daughter of 90Sr with a half-life of 64 h) beta rays; for this purpose scanning-bed geometry enclosed in a special shielding room is used. Analyses of 137Cs and 40K are accomplished at the same time with the same detector by the measurement of their photopeaks (photons with energy ranges 620-740 keV and 1400-1580 keV, respectively). The author of the first dual crystal used for direct measurement of internally deposited radionuclides was Dr. G. Laurer. This crystal design directly led to the development and use of 'phoswich' detectors.
'Phoswich' detectors to be used in SICH-9.1M for measurement of the bremsstrahlung of 90Y beta rays as well as electronic scheme and shielding room were designed by the group of scientists headed by Prof. Yu. Belle.
The principal features of the WBC are illustrated in Figure 1. The shielding room is made of cast-iron rings with a wall thickness of 200 mm. The inner surface has linings of lead (5 mm), cadmium (1 mm), and copper (2 mm). Four phoswich detectors located in central vertical plane are fixed on binding joists. During measurement the person lying on the bed is moved through the detector array over a scanning length of 2 m.
Original WBC has been worked over 24 years from 1974 to 1997. During this long-term period more than 38,000 measurements were carried out on more than 20,000 persons by the WBC group under head of V. Kozheurov (Kozheurov, 1994) In 1998, a question on modification of the original SICH-9.1 has been raised in order to provide for the continuation of the individual body-burden monitoring program. In a improved WBC it was decided to use the same shielding room and the same geometry of measurements (scanning-bed), as well as the same type of detectors (phoswich).
In 2002, Dr. G. Laurer and Dr. D. Hickman proposed a new modification of phoswich detectors to be used in the improved WBC. These detectors were purchased from Saint-Gobain Cristaux & Detecteurs (France). In 2002, Drs. Laurer and Hickman came to URCRM to test these detectors.
A new electronic scheme for the improved WBC was proposed and developed by Dr. N. Bougrov and V. Farafontov (Bougrov and Farafontov, 2004). Modular electronics for the improved WBC was produced by EG&G ORTEC (USA).
Figure 2 shows the electronic scheme and its major components. A photo of the modular electronics is shown in Figure 3.
Fig.2. Electronics scheme of the improved WBC.
The major components shown in Fig.2 are: 1) Phoswich detector (Saint-Gobain Cristaux & Detecteurs, France); 2) EG&G ORTEC modular electronics (USA):
|Fig.3. SICH-9.1M WBC electronics. A spectra is shown on computer screen.|
Figure 4 shows a general view of the whole body counter: exterior view of the WBC is shown on the left panel and interior view with an array of four phoswich detectors placed in the central vertical plane is shown on the right panel.
|Fig.4. General views of the whole body counter.|
Calibration with an anthropomorphic phantom of an adult person, designed by Dr. A. Kovtun (Kovtun et al., 2000b), with tissue substitutes (cold-cured epoxy resin products with different mass densities) adequately simulating the attenuation properties of human tissues with uniformly distributed 90Sr course in the skeleton is now underway. Figure 5 shows the anthropomorphic phantom prepared to be measured.
|Fig.5. Anthropomorphic phantom prepared to be measured.|
A solid whole-body phantom set assembled of right-angled polyethylene units and rod radionuclide sources of 137Cs and 40K inserted into them will be used for calibration for these radionuclides. This phantom set simulates body characteristics of children and of adults of different weights (Kovtun et al., 2000a).
The whole body counter (WBC) SICH-9.1M has been certified by the Federal Technical Control and Metrology Agency and entered into the State registry of measuring instruments under №31185-06, on March 14, 2006, and approved for application in the Russian Federation. The Method for measuring body burdens from radionuclides of 90Sr, 137Cs and 40K using WBC SICH-9.1M has also been certified on June 30, 2006.
List of certificates
|Fig.6. Certificates of SICH-9.1M.|
The measurements of Urals population using modernized Whole body counter SICH-9.1M have been resumed since the 1st of July 2006..
The modernization of the WBC is funded by the US Department of Energy's Office of Health Programs (JCCRER Project 1.1). Participants of Project 1.1 are thankful to Dr. G. Laurer and Dr. D. Hickman for valuable help in testing of the new phoswich detectors and to Dr. A. Kovtun for providing phantoms for calibration of the improved WBC.
|© 2007.||October 10, 2007 - Biophysics Lab, URCRM|