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ORIGINAL ARTICLE
Year : 2016  |  Volume : 39  |  Issue : 4  |  Page : 190-193  

Development of an ionization chamber-based high sensitivity detector for the measurement of radiation dose from X-ray whole body scanners


Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India

Date of Web Publication13-Feb-2017

Correspondence Address:
Sunil Kumar Singh
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.199979

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  Abstract 

Dose from X-ray-based transmission type whole-body scanner (WBS) is very small and its measurement is a regulatory requirement as recommended by ANSI/HPS N43.17-2009. Measurement of dose of the order of 25 µ R per scan (received by the person screened) requires very sensitive detector/instrument whose response is energy independent. Such systems are not commercially available. In view of this, a large volume, high sensitivity, ionization chamber (IC) was designed and developed for its use in the measurement of reference effective dose from transmission type X-ray-based WBSs. The IC has thin aluminum wall and was tested for direct X-ray beam from 120 to 180 kV. The measured sensitivity for X-ray spectrum from 120 to 180 kV was found to be 41.75 pC/µR with a standard deviation of ± 0.53%. The field uniformity at the measurement distance of 5 m was within ± 5%.

Keywords: Ionization chamber, whole-body scanner, X-ray dose


How to cite this article:
Singh SK, Tripathi SM, Shaiju L, Sathian V, Kulkarni MS. Development of an ionization chamber-based high sensitivity detector for the measurement of radiation dose from X-ray whole body scanners. Radiat Prot Environ 2016;39:190-3

How to cite this URL:
Singh SK, Tripathi SM, Shaiju L, Sathian V, Kulkarni MS. Development of an ionization chamber-based high sensitivity detector for the measurement of radiation dose from X-ray whole body scanners. Radiat Prot Environ [serial online] 2016 [cited 2020 Feb 22];39:190-3. Available from: http://www.rpe.org.in/text.asp?2016/39/4/190/199979


  Introduction Top


Use of walk through metal detectors and frisking of personnel are the conventional methods used by the security forces to detect illegal carriage of dangerous materials at the airports, seaports, railway stations and other sensitive places. Use of plastic explosives, drug or illegal carriage of dangerous items concealed under cloth or body cavities has increased many folds in the recent past which in many cases is not possible to detect by conventional methods. In view of present security scenario, such conventional methods need augmentation. One of such systems capable to overcome the above difficulties is to use X-ray-based whole-body scanner (WBS), either transmission type or backscatter type, depending upon the nature of requirement. While using these WBSs, the person being scanned can be monitored by following international standards which recommends certain dose limits. To check the compliance of these recommended dose limits, the dose per scan received by the person (from these WBSs) needs to be measured.

Conventionally, Geiger-Müller or Scintillation detector (operated in pulse mode)-based radiation monitoring instruments are in use to measure low-intensity radiations. These detectors show prominent energy dependence at lower energies, mostly below 200 keV energy. Since X-ray comprises of spectrum of energies, the instruments based on these types of detectors do not suit for the measurement of such low X-ray doses. However, ionization chamber (IC) detector-based instrument, operated in DC mode, is better suited for measurement of doses as it shows an energy independent response in the X-ray energy range of our interest. Therefore, an IC, having very high sensitivity, has been designed, developed and fabricated for measuring the extremely low-exposure X-ray fields (~few µR) produced by a scanning X-ray beam over a large area. A methodology has been developed to measure exposure per scan using large volume ICs. This value of exposure will be used to evaluate reference effective dose as per the recommendations of ANSI/HPS N43.17-2009 standard [1] for its compliance.


  Materials Top


Measurement of radiation field in terms of exposure/rate due to spectrum of low-energy X-ray radiation needs special considerations as the photoelectric interaction is mainly responsible for the signal generation. Studies with spherical and cylindrical ion chambers in 15 keV X-ray beams showed that the chamber sensitivity at low energy strongly depends on curvature of the chamber wall, especially in thick-walled chambers.[2] To overcome the above-mentioned problems, thin- and plane-walled ICs are used [2] for the measurement of exposures from low-energy photons. Thus, a thin aluminum-walled (thickness ~13 µm) IC of dimension 30 cm × 30 cm × 150 cm (volume ~135 L) was designed and fabricated [Figure 1]a. Normally transmission type X-ray-based WBSs uses fan beam of small thickness (few mm). These scanners uses ~155 kV X-rays to scan an area of ~210 × 75 cm 2 (i.e., ~7 feet × 2.5 feet). The dose rate varies with height of the fan beam; therefore, the chamber dimensions were selected in such a way to represent an average dose in the height of the detector. 10 mm × 10 mm aluminum rods were used to make a frame of dimension 30 cm × 30 cm × 150 cm. One millimeter thick aluminum plates were fixed at four opposite wall, with areal dimension of 30 cm × 150 cm (two numbers, opposite walls) and 30 cm × 30 cm (two numbers, one top and one at bottom) [Figure 1]a. A commercially available aluminum foil of ~13 µm was wrapped around two remaining opposite walls with dimension 30 cm × 150 cm, to construct its wall. The central electrode is made up of 1.6 mm diameter copper wire. Central electrode and guard ring connections were made through a BNC connector installed in the top plate and a lug is attached to one of the rod for getting the connection of triaxial cable connected to electrometer. The chamber was designed in such a way that it can be dismantled and can be transported in a suitcase and can be quickly assembled. The fabricated IC was assembled [Figure 1]b and coupled with a reference class electrometer [3] (PTW UNIDOS) for its X-ray response characterization.
Figure 1: (a) Sketch of 135 L ion chamber. (b) Photograph of 135 L chamber

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  Experimental Top


The developed IC was tested for its sensitivity using direct X-ray beams of 120 to 180 kV generated from a dosimetric grade X-ray machine established as a calibration facility in Radiation Standards Section, Bhabha Atomic Research Centre. The X-ray machine can be tuned from 15 to 320 kV. Power rating of the X-ray machine is 4.2 kW. A parallel plate Free Air IC (FAIC) with plate separation of 28 cm and aperture of 10 mm diameter [Figure 2] was used for standardization of direct X-ray beam. It has a sensitive volume of 7.85 cc and is an absolute standard for the measurement of exposure due to X-ray beams. A 600 cc IC (Saint-Gobain make, model NE 2575C) [Figure 3] with sensitive volume of 600 cc was calibrated against FAIC as a secondary standard for the measurement of exposure due to direct X-ray beam. It measures unidirectional radiation beam incident normal to the window, for photon energies ranging from 0.01 to 2 MV, with appropriate fitted windows. A reference class electrometer (PTW UNIDOS) having a measurement least count of 1 fA was used as low-level current measuring device with all the three ICs mentioned above. The electrometer has an inbuilt high voltage unit of ±400 V, which can be tuned in a multiple of ±50 V.
Figure 2: Free air ionization chamber (an absolute standard for X-ray output measurement)

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Figure 3: 600 cc ionization chamber used for calibration of 135 L chamber

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Sensitivity measurement of designed ionization chamber

The output of the X-ray machine in terms of exposure rate was standardized at various kilovolts, applied to the X-ray tube, using FAIC (an absolute standard for X-ray output measurement) coupled with a reference class electrometer. A 600 cc thin window IC was calibrated against the output measured by FAIC using substitution technique.[4] The assembled IC of 135 L, coupled with electrometer, is then placed in front of the X-ray machine (substitution technique) and calibrated against 600 cc IC. The 600 cc IC is calibrated at a distance of 200 cm while 135 L chamber was calibrated at a distance of 500 cm. Ideally a distance more than 750 cm is required to maintain field uniformity better than 1% but due to room size constraint, measurement was done at a distance of 500 cm. At this distance of 500 cm, field size uniformity was measured for entire length of 135 L chamber (i.e., for a height of 150 cm) using 600 cc chamber and is found to be within ±5%. A laser-based alignment system was used to align the center of the each IC with the focal spot of the X-ray machine. The exposure rate linearity along with collection efficiency of 135 L IC was measured up to ~300 mR/h of exposure rate, against 600 cc chamber [Table 1].
Table 1: Linearity response of 135 L ionization chamber at 400 V applied potential

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  Results and Discussions Top


It is normally observed that X-ray-based transmission type WBSs use 155 kV X-ray beam for scanning; thus the sensitivity of the IC was measured at high voltage ranging from 120 to 180 kV and the average sensitivity was found to be 41.75 pC/µR with standard deviation of 0.53% [Table 2]. The collection efficiency of this chamber was tested using two voltage method.[5] The exposure linearity response of this detector, with an applied electrode potential of 400 V, was found linear, with charge collection efficiency better than 98%, up to exposure rate of 40 mR/h. From [Table 1], it can be seen that this detector along with electrometer can even be used for the measurement of 200 mR/h with corrected charge collection efficiency (which is better than 90% at 200 mR/h). This detector coupled with UNIDOS Universal Dosemeter, having charge measurement range starting from 250 fC (least count of 10 fC) for radiation protection,[3] can measure integrated exposure of even 0.1 µR.
Table 2: 135 L chamber response at various X--ray beam qualities

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This detector along with sensitive electrometer can be used for the measurement of exposure per scan due to transmission type X-ray-based (~155 kV) WBSs which will be further used for the calculation of reference effective dose [Equation 1] per scan as specified by ANSI-2009.[1]



Where ERef = Reference effective dose in rem; X = Measured exposure in Roentgen (R); CR = Conversion factor in rem/R.

The value of CR can be evaluated as per the recommendations of ANSI mentioned below:

CR = 0.110 × HVL of X-ray beam in mm of Al.

Or

= 1.00, whichever is smaller.

It can be concluded that a very high sensitive large volume IC, whose sensitivity remains constant for a large range of photon energies, can be used for the measurement of average dose per scan delivered by X-ray-based transmission type WBS to check its compliance for radiation protection limits as per the international standards.

Acknowledgment

The authors sincerely acknowledge the contributions of Shri A. K. Mahant (Ex-BARC Scientist) in the study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

1.
ANSI/HPS N43.17. Radiation Safety for Personnel Security Screening Systems Using X-ray or Gamma Radiation; 2009.  Back to cited text no. 1
    
2.
Mahant AK, Singh SK, Panyam VS. Development of ion chambers for the measurement of low energy synchrotron radiation. Nucl Instrum Methods Phys Res A 2009;601:354-7.  Back to cited text no. 2
    
3.
User's manual for PTW UNIDOS Model T10005, PTW Freiburg GmbH, Germany [2003].  Back to cited text no. 3
    
4.
NCRP Report 112, Calibration of Survey Instruments Used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination; 1991.  Back to cited text no. 4
    
5.
Attix FH, Introduction to Radiological Physics and Radiation Dosimetry ch. 12. John Wiley & Sons: New York; 1986. p. 335.  Back to cited text no. 5
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]



 

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