|Year : 2017 | Volume
| Issue : 2 | Page : 99-102
In-house development of automatic distancing system for calibration check of portable radiation survey instruments
Wiquar Ahmad, SR Mitra, AK Mitra, V Parashar, CS Mahala, SD Geete, SH Patil
Department of Atomic Energy, Tarapur Atomic Power Station 1 and 2, Tarapur Maharashtra Site, NPCIL, Palghar, Maharashtra, India
|Date of Submission||23-Feb-2017|
|Date of Decision||04-Apr-2017|
|Date of Acceptance||03-May-2017|
|Date of Web Publication||13-Jul-2017|
Health Physics Unit, Tarapur Atomic Power Station 1 and 2, Tarapur Maharashtra Site, NPCIL, Palghar - 401 504, Maharashtra
Source of Support: None, Conflict of Interest: None
Calibration check of radiation survey instruments needs to be carried out periodically so as to verify that the calibration of instruments holds good during its use. Multipoint calibration check of every radiation survey instruments is carried out on quarterly basis. Conventionally, calibration check was done by aligning the source or the instrument manually at different distance from each other to obtain readings at different dose rates or radiation fields. This incurs radiation exposure to the radiation worker, which is avoidable. An automatic distancing system was developed in-house at Tarapur Atomic Power Station - 1 and 2 which minimized the exposure of handling radiation source by automatically aligning the source to the instrument and distance between them remotely and also increased the precision in measurement.
Keywords: Automatic distancing system, calibration, exposure, source
|How to cite this article:|
Ahmad W, Mitra S R, Mitra A K, Parashar V, Mahala C S, Geete S D, Patil S H. In-house development of automatic distancing system for calibration check of portable radiation survey instruments. Radiat Prot Environ 2017;40:99-102
|How to cite this URL:|
Ahmad W, Mitra S R, Mitra A K, Parashar V, Mahala C S, Geete S D, Patil S H. In-house development of automatic distancing system for calibration check of portable radiation survey instruments. Radiat Prot Environ [serial online] 2017 [cited 2020 Jul 13];40:99-102. Available from: http://www.rpe.org.in/text.asp?2017/40/2/99/210580
| Introduction|| |
The regulatory requirement  for calibration states that the radiation survey instruments should be calibrated periodically. The frequency of calibration should be decided by the user. At Tarapur Atomic Power Station (TAPS), radiation survey instruments are calibrated quarterly by health physics unit and once in two year at RSSD, BARC.
The calibration check of radiation survey instruments is carried out at health physics calibration room on a platform. The desired distance between the source and instrument was achieved by moving the radioactive source manually with respect to the instrument. Based on measured and calculated reading, deviation from expected value was calculated. This process involved repeated relocation of source, instrument, and handling of source which incurred exposure. The source and instrument were kept on platform; therefore, error due to scattering  from hard surface was inherent. These shortcomings were eliminated by the development of an automatic distancing system (ADS) in which both source and the instrument are located about one meter above the horizontal surface, distance between source and instrument is adjusted remotely using a motorized system and handling of source is reduced only for placing the instrument on the ADS. This paper describes the basic construction functioning and working of ADS.
| Description of Automatic Distancing System|| |
ADS consists of four parts, instrument mounting platform, source moving mechanism, cameras for viewing distance and monitor reading, and workstation and associated software.
Instrument mounting platform
As shown in [Figure 1], the platform is designed in such a way that all types of available radiation survey instruments can be placed and aligned to the source. Groove and clamps are available for proper alignment and fixing. An 8.0 megapixel camera  for seeing monitor reading is fixed on platform through flexible gooseneck such that, it can be focused to see monitor scale as per instrument design. The platform can rotate 360° for polar response check of the instrument.
Source moving mechanism
The source moving mechanism is a source holder assembly mounted on a board which slides on “C” channel with the help of chain and sprocket arrangement as shown in [Figure 2]. Chain sprocket arrangement can have backlash error; hence, the pointer showing the distance and source has a rigid link as they are part of the source platform thus backlash error if present does not have any effect on the distance shown by the pointer. The source platform is moved till the desired distance reading is achieved. The whole system is driven by a servo motor  (1 RPM) which is operated by a remote control. Minimum measurable distance is 1 mm and maximum distance between source and instrument is 3 m. A dummy source is used for detector center alignment which is replaced by the actual source after the alignment is completed. Source holder is equipped with 8.0 megapixel camera for distance scale reading. As shown in [Figure 3], the camera is focused on a pointer fixed to the holder assembly above the scale showing the distance reading.
Workstation and software
The workstation comprises display where output of the cameras, i.e., instrument scale reading camera and source holder assembly and pointer camera are displayed through a freeware application named “IP camera viewer version 1.34”. The software can display the outputs from maximum of four cameras simultaneously. The workstation is situated in different room from where the ADS can be operated remotely. Screenshots of software and camera view are shown in [Figure 4]a and [Figure 4]b.
Sources used are procured from Board of Radiation & Isotope Technology, and are pencil type Co-60 source encapsulated in a stainless steel capsule of outer diameter 7.5 mm and height 9 mm; the sources are certified for exposure rates by RSSD, BARC. The replica of the source is shown in [Figure 5]. The details of sources are tabulated in [Table 1].
|Table 1: List of certified cobalt-60 sources available in calibration facility|
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Instruments used in the facility
Portable and installed radiation monitor can be calibrated up to 20 mSv/h depending on its ranges. These radiation monitors are having two types of configuration, in the first type, detector is an integral part of the monitor and in the second type, detector is separate from the monitor. The configuration of the monitor becomes important when calibrating the instrument in high-radiation field as the circuit adjustment needs to be done with the instrument exposed to source. The details of various instruments used are tabulated given in [Table 2].
|Table 2: Radiation monitors calibrated using the facility during 2015 to 2016|
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| Operational Aspects of Automatic Distancing System|| |
The instrument to be calibrated is placed on instrument mounting platform and camera is adjusted such that it gives the proper resolved view of the monitor reading. Dummy source is mounted on source holder assembly and aligned to the detector. Source holder is tightened on the source top and source is lifted using source handling tongue and placed on source holding assembly in place of the dummy source. The distance between source and detector is varied remotely till it reaches desired distance as shown by source holder camera above the pointer. The monitor reading is noted from the other camera display and deviation if any from the expected value is calculated.
| Calibration Adjustment Using Mathematical Approach|| |
For microprocessor-based survey meters having the feature of entering calibration readings corresponding to expected exposure rate, the measurements are noted. Regression analysis with intercept set, a zero is performed for expected and observed exposure rate and modifying constant for existing calibration factor is found out by the slope of the regression line. A typical example is given in [Table 3]a. When the monitor is analog type, which is usually controlled by a series-parallel arrangement of potentiometer with one pot adjustment for each range, similar procedure is adopted. However, the pot adjustment is based on the fitted value rather than adjustment done using the conventional trial and error method. In case of log scale monitors, such as installed area radiation monitors, the analog circuit has dual pots for adjustment of gain and offset of amplifier, the regression can be performed to y = mx + c, where m indicates gain and c indicates offset adjustment. Unlike the conventional method of pot adjustment in the every decade by trial and error method, here a single-point adjustment helps calibration in the entire range of the monitor covering several decades. A typical example of installed high range area radiation monitor covering the entire range is shown in [Table 3]b.
|Table 3b: Typical calibration record of dual pot analog log scale monitor (GM tube, 0.001-1 mSv/h)|
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| Conclusions|| |
Introduction of ADS not only gives the confidence in calibration check process but also reduces exposure and inherent errors. Earlier repeated attempts were required for a single instrument to get desired values. With the implementation of ADS, entire calibration check gets completed in a single attempt. The system was developed with in-house resources and at nominal expenditure.
We are thankful to Shri A. K. Singh, Site Director, Tarapur Maharashtra Site, Shri V. S. Daniel, Station Director, TAPS 1 and 2 for encouraging the project and overseeing its implementation. We would also like to thank senior engineers Shri R. Murali, Shri S. K. Chourasia, Shri K. V. S. N. Murthy, and Shri R. K. Sahu for giving valuable resources, support, and making this effort a success.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
Atomic Energy (Radiation Protection) Rules; 2004.
Linear Distancing System for Calibration of Radiation Detectors, BARC Newsletter; 2007;278:9.
IAEA. Calibration of Radiation Protection Monitoring Instruments. IAEA Safety Report Series No. 16. Vienna: IAEA; 2000.
iBall Face2Face C8.0 (Rev. 3.0) Web Camera Specification Data Sheet.
Synchronous Motor MIS-4 Data Sheet Copal Co. LTD Tokyo, JAPAN.
BARC/RSSD/RSS/CAL/C-948/2016 Dt; 13 December, 2016.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4]