ARTICLE IN PRESS

Applied Radiation and Isotopes 68 (2010) 1173–1176

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Applied Radiation and Isotopes

0969-80

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/apradiso

Technical note

Magnetic removal of electron contamination for 60Co panoramic gamma rayexposure—Investigations with CaSO4:Dy and LiF based dosimeters

Munish Kumar a,�, G. Sahani b, G. Chourasiya a

a Radiological Physics and Advisory Division, Bhabha Atomic Research Centre (BARC), Mumbai 85, Indiab Radiological Safety Division, Atomic Energy Regulatory Board, Mumbai 94, India

a r t i c l e i n f o

Article history:

Received 13 March 2009

Received in revised form

11 December 2009

Accepted 11 December 2009

Keywords:

Thermoluminescence dosimetry

Electron contamination

Magnetic field

43/$ - see front matter & 2009 Elsevier Ltd. A

016/j.apradiso.2009.12.034

esponding author.

ail address: munish_pms@rediffmail.com (M.

a b s t r a c t

Electron contamination from a sealed 60Co radiation source has been investigated comprehensively

using a CaSO4:Dy based TLD badge and LiF crystals. It has been found that due to electron

contamination, the thermoluminescence (TL) detectors exhibit over response which can be corrected by

applying a magnetic field. It has also been found that for a source-to-dosimeter distance of 50 cm, the

ratio of the TL readouts of the third to first discs of the TLD badge reduces from �1.5 to �1.00 after

applying a magnetic field. Hence detectors which are sensitive to electrons as well as photons, and are

capable of distinguishing them, can lead to an erroneous measurement. This happens because the

contribution due to electron contamination interferes with pure gamma calibration. The study is

helpful in establishing accurate calibration and appropriate correction factors for personnel monitoring

carried out using CaSO4:Dy based TLD badge.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

For sealed 60Co radiation sources used in routine dosimetrycalibration and industrial (radiography gamma camera) applica-tions, the interaction of photons via photoelectric and Comptoneffects with the source shielding and surrounding air can lead tothe production of electrons popularly known as electron con-tamination. Similarly, electron contamination have been observedfor telecobalt machines and other high energy photon andelectron sources (medical/industrial linear accelerators andelectron synchrotrons) (Nilsson, 1985; Sjogren and Karlsson,1996 and Sahani et al., 2008).

In India, the radiation doses due to x, b and g radiations aremeasured using CaSO4:Dy based TLD badge which have three TLdiscs each having a thickness and diameter of 0.8 and 13.3 mm,respectively. These discs are clipped on Ni-plated aluminium cardwhich is further enclosed in a cassette having three filters. Thefirst dosimeter disc is surrounded by 1 mm Cu and 1 mm Al filters(total thickness ffi1160 mg cm�2), whereas the second dosimeterdisc is surrounded by Perspex (180 mg cm�2). The third dosimeterdisc is open. Further details about the TLD badge can be had fromVohra et al. (1980) and Munish et al. (2008).

The fact that for pure gamma radiation exposures from 137Csand 60Co sources, the TL readouts of the three discs of the TLDbadge used in our countrywide personnel monitoring are

ll rights reserved.

Kumar).

expected to show nearly same readings within statistical fluctua-tions, i.e. D1ffiD2ffiD3, where D1, D2 and D3 are the values of theTL counts of the three discs of the TLD badge. Any deviation fromthe standard readout, i.e. D1ffiD2ffiD3 may indicate the presenceof interfering radiation. In fact, the design of the TLD badge is suchthat different ionizing radiations such as gamma and beta-rayscan be separated/distinguished and the doses could be measured.It is also worth mentioning that for exposures carried out with asealed 60Co radiation source, abnormal readout values of the threediscs of the TLD badge has been observed earlier by manyworkers; however, no attempts were made to quantify andremove electron contamination (Popli et al., 1985; Lakshmananet al., 1986; Bakshi et al., 2003). A similar problem was alsoencountered while participating in internal quality assurancechecks when exposures were carried out with sealed 60Co source.

In this study, efforts have been made to study and remove theelectron contamination for panoramic gamma ray exposuresituations by the application of magnetic field.

2. Materials and methods

In this study, the TLD cards having sensitivity variation o75%(1s) were used. The irradiations of the TLD cards enclosed in thecassette were carried out at a height of 1.20 m from the ground usingsealed 60Co source. Initially the experiment was carried out at sourceto dosimeter distance of 50 cm as same distance is used in ourroutine calibration of TLD badges and same is described in Fig. 1.Further the study was also carried out for various source to

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M. Kumar et al. / Applied Radiation and Isotopes 68 (2010) 1173–11761174

dosimeter distances varying from 15 to 100 cm. The irradiateddosimeters were read on a calibrated hot gas TLD badge reader aftera waiting period of 7 days which minimizes the interference from thelow temperature TL peak. Details about hot gas TLD badge reader canbe had from Kulkarni et al. (2000) and Kumar et al. (2005).

The experiment was also repeated in the presence of parallelplate magnets (magnetic field strength, B=0.025 T) under variousconditions viz magnetic field applied: (i) after the source only and

Fig. 1. Schematic diagram of the exposure setup. The electron contamination from the

form of dotted lines represent another case when the magnetic field is applied to the

Table 1Readout pattern of the TLD badge under various exposure conditions for 137Cs and 60C

Exposure

Source to dosimeter distance=50 cm

TLD badge (card in cassette)

5 mm Perspex buildup to source

10 mm Perspex buildup to source

5 mm Perspex buildup each to source and TLD badge

10 mm Perspex buildup to source and 5 mm Perspex build up to TLD badge

Source without any buildup and TLD badge covered with 5 mm Perspex buildup

Source without any buildup and TLD badge covered with 10 mm Perspex buildup

D1, D2 and D3 are the values of the TL readouts of the three discs of the TLD badge.

Table 2Readout pattern of the TLD badge under various exposure conditions for 137Cs and 60C

Exposure

Source to dosimeter distance=50 cm

TLD badge (card in cassette)

5 mm Perspex buildup to source

10 mm Perspex buildup to source

5 mm Perspex buildup each to source and TLD badge

10 mm Perspex buildup to source and 5 mm Perspex build up to TLD badge

Source without any buildup and TLD badge covered with 5 mm Perspex buildup

Source without any buildup and TLD badge covered with 10 mm Perspex buildup

(ii) before the TLD badge only. This was carried out to verify thefact that the observance of abnormal values of the three discs ofthe TLD badge is due to scattered low energy photons orcontamination electrons. If it is due to photons, then there shouldnot be any effect after the application of magnetic field as photonsare neutral. However, if it is due to electrons, then they willexperience the deflection caused by the magnetic force and willbe unable to reach at the dosimeter.

source is removed by applying a magnetic field. The magnetic plates shown in the

TLD card only.

o irradiations (no magnetic field applied).

137Cs source 60Co source

D3/D1 D2/D1 D1 D3/D1 D2/D1 D1

1.14 1.09 1.00 1.49 1.26 1.00

1.10 1.09 1.00 1.48 1.24 1.00

1.10 1.11 1.00 1.50 1.25 1.00

1.00 1.06 1.00 1.26 1.19 1.00

1.06 1.11 1.00 1.17 1.14 1.00

1.06 1.07 1.00 1.15 1.15 1.00

1.10 1.10 1.00 1.18 1.16 1.00

o irradiations carried out in the presence of magnetic field.

137Cs source 60Co source

D3/D1 D2/D1 D1 D3/D1 D2/D1 D1

1.13 1.10 1.00 1.00 1.05 1.00

1.09 1.08 1.00 1.02 1.03 1.00

1.11 1.10 1.00 0.99 1.09 1.00

1.02 1.08 1.00 0.97 1.11 1.00

1.07 1.10 1.00 1.00 1.12 1.00

1.04 1.09 1.00 0.98 1.10 1.00

1.09 1.1 1.00 1.03 1.10 1.00

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M. Kumar et al. / Applied Radiation and Isotopes 68 (2010) 1173–1176 1175

The same experiment was also carried out by keeping LiF, TLD-100Harshaw single crystals on Teflon discs in the cassette and theexposures were carried out with and without applying a magneticfield. Before performing the experiment, the TLD chips were tested forsensitivity variation by exposing to a standard dose of 0.50 Gy. Onlythose chips were used in the experiment, which have spread insensitivity o72% (1s). The TL spectrum was recorded usingcalibrated TL reader. The results obtained were similar to thoseobtained using CaSO4:Dy based TLD badge. In addition, the experi-ment was also carried out by exposing CaSO4:Dy discs and LiF crystalsindividually, with and without applying magnetic field. Same studieswere also carried out using 137Cs radiation source.

3. Results and discussion

The TL readout pattern of the three discs of the TLD badge i.e.D1, D2/D1 and D3/D1 observed at a distance of 50 cm for 137Cs and60Co sources are presented in Table 1 for various exposure

00.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e be

ta d

ose

Distance (cm)20 40 60 80 100

Fig. 2. Variation of electron contamination/beta dose (relative) with source to

dosimeter distance.

Table 3Readout pattern of LiF dosimeters kept on Teflon discs of the TLD Badge for 137Cs and

Exposure

Source to dosimeter distance=50 cm

TLD badge (card in cassette)

5 mm Perspex buildup to source

10 mm Perspex buildup to source

Source without any buildup and TLD badge covered with 10 mm Perspex buildup

Table 4Readout pattern of LiF dosimeters kept on Teflon discs of the TLD Badge for 137Cs and

Exposure

Source to dosimeter distance=50 cm

TLD badge (card in cassette)

5 mm Perspex buildup to source

10 mm Perspex buildup to source

Source without any buildup and TLD badge covered with 10 mm Perspex buildup

conditions, whereas in Table 2, the readout pattern observed inthe presence of magnetic field is shown. In Tables 1 and 2, thenormalization of the readouts of three discs is carried out, whichin this case is with respect to the value of D1 and D1, D2/D1 andD3/D1 is described here as readout pattern. From Table 1, it isseen that the readout pattern of the TLD badge for normalexposure carried out at 50 cm distance from the 60Co source isD1ffi1:0, D2ffi(1.25–1.30)D1 and D3ffi(1.40–1.50)D1, whereasfor magnetically filtered beam, the readout pattern is D1=1.0,D2r1.15D1 and D3ffi1:0D1.

It is also worth to mention that for the exposures carried outby keeping 5 and 10 mm Perspex buildup plates around thesource, the abnormal readout pattern is obtained. This is due tothe reason that the electrons generated from the source shieldingare cut by the 5/10 mm thick Perspex; however, due to theinteraction of gamma photons with Perspex, the electrons areemitted from the outer layer of the Perspex which leads to anabnormal readout pattern. Further if a 10 mm thick Perspex sheetis placed before the TLD badge, then normal readout pattern isobtained, i.e. D1ffiD2ffiD3. This is due to the fact that the numberof photons interacting with the Perspex at 50 cm distance is verysmall in number (follows inverse square law) to cause anydeviation from normal readout pattern. It is also worth mention-ing that for a sealed 60Co radiation source, the electroncontamination is observed up to 80 cm distance (Fig. 2) and isalso supported by the fact that the maximum energy of electronsgenerated by photoelectric or Compton scattering is �1 MeV andcan have range of about 3 mm and 3.0 m in Perspex and air,respectively. Similar results are obtained with LiF TL crystals(Tables 3 and 4) when irradiations are carried out with andwithout applying a magnetic field.

4. Conclusions

(i)

60Co

60Co

The electron contamination for 60Co exposures arises dom-inantly from source shielding and decreases with increasingdistance between source and the dosimeter and disappearsafter nearly 80 cm distance.

(ii)

The introduction of Perspex sheets of varying thicknessaround the source does not result in any appropriate changesin electron dose at various distances because the shield itselfcontributes in the generation of electrons.

irradiations (no magnetic field applied).

137Cs source 60Co source

D3/D1 D2/D1 D1 D3/D1 D2/D1 D1

1.13 1.12 1.00 1.4 1.24 1.00

1.09 1.08 1.00 1.39 1.25 1.00

1.11 1.10 1.00 1.41 1.20 1.00

1.09 1.1 1.00 1.03 1.10 1.00

irradiations in the presence of magnetic field.

137Cs source 60Co source

D3/D1 D2/D1 D1 D3/D1 D2/D1 D1

1.13 1.12 1.00 1.00 1.11 1.00

1.09 1.08 1.00 0.97 1.10 1.00

1.11 1.10 1.00 0.99 1.09 1.00

1.09 1.1 1.00 1.01 1.10 1.00

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(iii)

Negligible electron contamination is observed for exposurescarried out with a 137Cs radiation source, whereas for 60Coexposure, significant electron contamination is observed. Hencethe difference observed in the three-disc readout pattern of theTLD badge, used in our countrywide personnel monitoringprogramme for 60Co exposures, is due to electron contamina-tion which can be removed by applying a magnetic field.

Acknowledgements

Shri H.S. Kushwaha, Director, Health, Safety andEnvironment Group (HS&EG), BARC and Dr. Y.S. Mayya, Head,RP&AD, BARC are the guiding force behind this work and theauthors are thankful to them for encouragement. The authors arealso thankful to Shri S.M. Tripathi and Liji Daniel of RadiationStandards Section (RSS), RSSD, BARC for providing the exposuresetup/facility as and when required. Discussions with Dr. A.S.Pradhan are gratefully acknowledged. Authors are thankful to theanonymous referees and Dr. Richard P. Hugtenburg for manyvaluable suggestions.

References

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