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ARTICLE
Year : 2010  |  Volume : 33  |  Issue : 4  |  Page : 213-215  

Assessment of 241 Am embedded in tissue using Phoswich and an array of HPGe detectors


Health Physics Division, Bhabha Atomic Research Centre, BARC Hospital, Anushaktinagar, Mumbai, India

Date of Web Publication1-Dec-2011

Correspondence Address:
Minal Y Nadar
Health Physics Division, Bhabha Atomic Research Centre, BARC Hospital, Anushaktinagar, Mumbai
India
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Source of Support: None, Conflict of Interest: None


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  Abstract 

Assessment of internal contamination of lung/skeleton/liver is required on a routine basis for occupational workers in fuel fabrication and reprocessing plants handling actinides. Assessment of lung/organ burden of actinides is carried out inside the totally shielded steel room whole body counter using Phoswich and array of HPGe detector. This paper describes studies carried out for calibration of the two detection systems for 241 Am source embedded at different depths in tissue equivalent perspex slabs. The calibration factors thus derived will be helpful in the estimation of embedded activity in the tissues of the radiation workers in cases of internal contamination due to wound in the workers handling actinides.

Keywords: Phoswich detector, array of HPGe detector, actinides embedded


How to cite this article:
Nadar MY, Singh I S, Bhati S. Assessment of 241 Am embedded in tissue using Phoswich and an array of HPGe detectors. Radiat Prot Environ 2010;33:213-5

How to cite this URL:
Nadar MY, Singh I S, Bhati S. Assessment of 241 Am embedded in tissue using Phoswich and an array of HPGe detectors. Radiat Prot Environ [serial online] 2010 [cited 2021 Aug 4];33:213-5. Available from: https://www.rpe.org.in/text.asp?2010/33/4/213/90477


  1. Introduction Top


The potential of internal contamination exists at different stages of the nuclear fuel cycle operations. Inhalation, ingestion, intact skin and the wound are the principle routes of entry of contaminants into the body. Therefore, monitoring of the workers of the nuclear industry is essential to assess, limit and control the internal contamination as it is required that doses resulting from such intakes by workers be evaluated and kept within the stipulated limits as specified by ICRP (ICRP, 78). Assessment of internal contamination of radiation workers due to actinides (Plutonium, Uranium isotopes and 241 Am) by direct method is based on the detection of low energy photons (LEP) accompanying their decays. The low values of lung/organ burdens of actinides required to be measured through the detection of low yield, low energies of the photons (E <100 keV) and their consequent severe attenuation within body tissues dictate the need for highly sensitive detection systems. Assessment of lung/organ burden of actinides is carried out inside the totally shielded steel room whole body counter using 20 cm dia and 1.2 cm thick Phoswich detector and an array of three HPGe detectors each of 7 cm dia and 2.5 cm thick. These systems are calibrated using realistic thorax JAERI phantom received under IAEA Coordinated Research Program. (Surendran et al. 1995, Pendharkar et al. 2008).

There are various parameters needed to be considered for accurate estimation of Lung burden of actinides, these are: spectral interference from other radionuclides ( 137 Cs), contamination in the wound and subsequent deposition in the lymph nodes and external contamination. This study discusses estimation of internal contamination in case of injection intake of actinides through a small injury or wound. There is a potential hazard of injury mainly on the hands while working with sharp tools inside the glove boxes and if the sharp edges are contaminated with radionuclides, this could lead to the activity getting embedded in the wound. Intake occurred by this pathway is known as 'Injection intake' (Graham et al., 1983). This paper presents the experimental measurements carried out to determine calibration factors (CFs) of the Phoswich and HPGe detectors for activity present at different depths in the tissue equivalent Perspex material and gives a methodology for the estimation of depth of activity. The results from this study will be useful for the assessment of internal contamination of 241 Am for injection intake of the activity through wound.


  2. Materials and Methods Top


2.1 Detection systems and measurement geometry

20 cm dia Phoswich detector: It consists of 1.2 cm thick NaI (Tl) as the primary and 5 cm thick CsI (Tl) as the secondary detector. It has a radiation entrance window of 0.5 mm thick Be. The detector is operated inside 20 cm thick steel room with graded z lining (Pb 3 mm + Cd 2 mm + Cu 0.5 mm) to reduce the background in low energy regions. Further reduction is achieved by applying Pulse Shape Discrimination (PSD) to cut off background of high energy photons in NaI (Tl) crystal by using CsI (Tl) as an anticoincidence detector (Pendharkar et al. 2008).

Array of HPGe detector: It consists of an array of three (7 cm dia. x 2.5 cm thick) LOAX HPGe detectors in one enclosure. The detector has 0.8 mm thick carbon entrance window. It has been commissioned as part of the up-gradation of in-vivo monitoring facility for measurement of internally deposited radionuclides in lungs and other organs of the radiation workers (Singh et al. 2008).


  3. Results and Discussion Top


For experimental measurements, 241 Am reference point source was kept in the tissue equivalent Perspex base plate (2cm thick) with a well of 2 cm dia and 4mm depth. The 241 Am spectra for the two detection systems were obtained for source plate to detector distance of 10 cm. The thickness of Perspex slabs over the source was varied from 1mm to 30 mm to simulate activity embedded in a tissue at different depths and the integrated counts in the various energy regions of 17.6, 26.3 and 59.5 keV were determined. This data is then used to evaluate CFs in different energy regions and at various depths of Perspex material from a 241 Am point source.

[Figure 1] gives calculated CFs (cpm/kBq) for 241 Am source as a function of thickness of the Perspex slabs for Phoswich and HPGe detectors. The 241 Am spectrum obtained using Phoswich detector shows a composite peak at ~18 keV corresponding to Np LX-rays of 13.9, 17.6, 21 keV due to it's poor resolution. As a result, [Figure 1]a shows CFs of Phoswich detector for 18 keV and 59.5 keV photons. The initial sharp fall in the CF values for Phoswich detector for 18 keV is due to the relatively much higher absorption of it's low energy (13.9 keV) X-rays in the intervening perspex slabs. However in case of HPGe detector, due to it's better resolution, the 241 Am spectrum shows resolved peaks for all the low energy X-rays and γ-rays. [Figure 1]b depicts CFs for three important energies (17.6, 26.3 and 59.5 keV) used in the calculation. Since the relative transmission of photons of different energies of 241 Am depend on the depth of the source under perspex slabs, the ratio of CFs in 59.5 keV to that of 26.3 keV or 17.6 keV regions can be used to determine source depth in tissue equivalent media. [Figure 2] depicts ratio of counts in 59.5 & 26.3 keV regions and 59.5 & 17.6 kev regions for an array of HPGe detector and ratio of counts in 59.5 and 18 keV regions for Phoswich detector as a function of depth.

For an actual case of 'Injection' intake of 241 Am through wound, monitoring should be carried out by positioning the wound at a 10 cm distance below the detector. The depth of the source can be determined by comparing the relevant ratios of count rates given in [Figure 2] with the ratio of count rates obtained from measuring the wound under the respective detector. The CFs depicted in [Figure 1] can then be used to assess the embedded activity in the wound of a subject for an evaluated depth.
Figure 1: Calibration factors of (a) Phoswich and (b) An array of HPGe detectors for 241Am reference source for different thicknesses of perspex slabs and energy regions

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Figure 2: Ratios counts in 59.5 & 26.3 keV regions and 59.5 & 17.6 kev regions for an array of HPGe detector and ratio of counts in 59.5 and 18 keV regions for Phoswich detector as a function of depth

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In case of injection intake involving mixture of radionuclides such as Pu and Am isotopes, it is not recommended to use the ratio of 59.5 and 17.6 keV for Phoswich or HPGe detector to estimate depth of activity. Since, it will lead to false depth evaluation due to contribution of 17 keV X-rays of 239 Pu in 17.6 keV of 241 Am region. Instead ratio of 59.5 and 26.3 keV is used for evaluating depth of activity. Accurate determination of depth is very important. [Table 1] gives error in the estimation of activity corresponding to an error in estimation of depth. It can be seen from the table that error in the evaluation of activity can lead to overestimation by 23.3) or underestimation by 16% corresponding to an error of ±1 cm in the estimation of depth of the activity.
Table 1: Error estimation of 241Am activity at 1 cm tissue depth using HPGe detector

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  4. Conclusions Top


The potential of internal contamination exists at different stages of the nuclear fuel cycle operations. There is always a potential hazard of injury while working with sharp tools inside the glove boxes. If the sharp edges contaminated with radionuclides lead to pricks or small cuts, activity can get embedded inside the tissues. Therefore, in this paper, calibration factors of Phoswich and HPGe detectors for 241 Am were evaluated for activity embedded in tissue equivalent material. The factors thus evaluated will lead to the accurate estimation of depth and embedded activity in the wound.


  5. References Top


  1. Graham S.G., Kirkham S.J. (1983), Identification of 241Am in the Axillary Lymph nodes with an Intrinsic Germanium Detector, Health Physics, Vol. 44, suppl No.1, 343-352.
  2. ICRP publication 78. (1997), Individual Monitoring for Internal exposure of workers.
  3. Singh I. S., Nadar M.Y, Kalyane G. N., Bhati S. and Pendharkar K.A. (2008), A Sensitive Steel Room Whole Body Counter with three LOAX HPGe Detectors for in vivo monitoring of personnel for actinides. Rad. Prot.Environ. Vol. 31(1/4), 379-381.
  4. Surendran, T., Haridasan, T.K., Sharma, R.C. and Krishnamony, S. (1995), Experiences at Trombay in Monitoring Actinide Intakes by Occupational Workers by Direct External Counting. Rad. Prot. Dosim. 59, No.1, 15-24.
  5. Pendharkar K. A., Bhati S., Singh I.S., Sawant P. D., Satyabhama N., Nadar M. Y., Vijaygopal P., Patni H.K., Kalyane G.N., Prabhu S. P., Ghare V. P. and Garg S.P. (2008), Upgradation of Internal Dosimetry Facilities at BARC Trombay. BARC Newsletter Issue No 296, 9-23.



    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1]



 

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Abstract
1. Introduction
2. Materials and...
3. Results and D...
4. Conclusions
5. References
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