Calibration of Optichromic Dosimeters for Routine Radiation ...

AdMet 2012 Paper No. RM 001
Calibration of Optichromic Dosimeters for
Routine Radiation Processing Dosimetry
Sachin G.V. Mhatre, S.H. Shinde and Sandip Mondal
Radiation Safety Systems Division
Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085.
E-mail: sachingm@barc.gov.in
Abstract: Dosimetry is crucial for radiation processing, which involves large variation
in doses and dose rates from photon and electron sources. The commercial viability
of processing is directly dependent on the accuracy of the dose measuring system.
Generally, proven dosimetry systems are widely used to perform radiation measurements
in development of new processes, validation, qualifi cation and verifi cation of established
processes. Proper calibration and traceability of routine dosimetry systems to standards
are crucial to the success of many large-volume radiation processes. Recently, M/s.
Innova Agri BioPark Ltd., India had procured optichromic dosimeters to be used as
routine dosimeters for mango irradiation. As these dosimeters were imported from nonequatorial
country, its dose response calibration needed validation or re-calibration
for the hot and humid conditions generally encountered in our country. Hence these
dosimeters were calibrated by RSSD, BARC using a calibrated gamma chamber 900
in the dose range of 200 to 2000 Gy. This dose response calibration was used by
M/s Innova for dose estimation in actual irradiation conditions of the food irradiation
plant during mango irradiation. The doses estimated were further verifi ed in reference
to alanine ESR dosimeter, a reference standard, placed along with the optichromic
dosimeters during the mango irradiation. The maximum variation found between the
dose measured by optichromic and alanine dosimeters was within ±10%, which is
acceptable as per the AERB’s recommendation for the dose verifi cation exercise.
Keywords: Calibration, optichromic dosimeter, Fricke dosimeter, gamma chamber 900
1. INTRODUCTION
Dosimetry is crucial for radiation processing,
which involves large variation in doses and
dose rates from photon and electron sources.
The commercial viability of processing is
directly dependent on the accuracy of the
dose measuring system. Generally, proven
dosimetry systems are widely used to perform
radiation measurements in development of
new processes, validation, qualifi cation and
verifi cation of established processes. Proper
calibration and traceability of routine dosimetry
systems to standards are crucial to the success
of many large-volume radiation processes [1].
Recently, M/s Innova Agri BioPark Ltd.,
India had procured optichromic dosimeters
to be used as routine dosimeters for mango
irradiation. As these dosimeters were imported
from non-equatorial country, its dose response
calibration needed validation or re-calibration
due to the hot and humid conditions generally
encountered in our country. Eventually, these
dosimeters were calibrated by RSSD, BARC
using a gamma chamber 900 initially calibrated
by Fricke dosimeter, a reference standard [2].
Dosimeters were calibrated in the dose range
of 200 to 2000 Gy.
This dose response calibration was further
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used by M/s. Innova for dose estimation
in actual irradiation conditions of the plant
during mango irradiation. These doses
were verifi ed in reference to alanine ESR
dosimeter, a reference standard [3], placed
along with the optichromic dosimeters during
the dose verifi cation exercise. The maximum
variation found between the dose measured
by optichromic and alanine dosimeters was
within ±10%, which is acceptable as per the
AERB’s recommendations [4].
2. EXPERIMENTAL
2.1 Materials and methods
All reagents used were of analytical
reagent grade and were used without further
purifi cation. Preparation and dose estimations
of Fricke dosimeters were carried out as
recommended [2] . Alanine pellets were directly
placed in pre-cleaned polystyrene container
having dimensions 6.5 mm o.d., 32 mm height
and 3 mm wall thickness. For these dosimeters,
dose measurements were made using e-scan
Bruker BioSpin alanine dedicated ESR
spectrometer. Specially designed perspex jig
were used during irradiation of optichromic
dosimeters in Gamma Chamber 900 and BI
2000. Absorbance measurements were made
using a calibrated [5] spectrophotometer.
2.2 Irradiation set-up for calibration of
optichromic dosimeters
Gamma Chamber-900 was used for
dose response calibration of the optichromic
dosimeters. Irradiations were done at the
center position of the irradiation volume. This
position was calibrated in terms of dose-rate
by using Fricke dosimeter [2].
1A
1B
1 Irradiation volume of G C-900
2 Build-up 3 Fricke dosimeter
4 Optichromic dosimeter 5 Perspex jig
1A Irradiation set-up for dose-rate calibration
using Fricke dosimeter
1B Set-up for irradiation of optichromic
dosimeters at calibrated position
Fig. 1: Irradiation set-up for calibration of
ptichromic dosimeter
2.3 Irradiation set-up for dose verification
exercise
During the dose mapping experiment,
single pellet was kept in the buildup and
nine such buildups were placed on each
of the three perspex sheets in each of the
dosimetry box as shown in Fig. 2. Mangoes
were packed in boxes having dimension 27.5
cm length, 37 cm width and 9 cm height. 20
such boxes in adjacent rows of ten each were
packed in the tote box having dimension 150
cm height, 76 cm length and 54 cm width.
Empty spaces around the stack of boxes
were fi lled with thermocol sheets. The bulk
density of mangoes was found to be about
0.43 g/cc. The total irradiation time was 5 hrs
14 mins and the irradiation temperature was
Specially designed perspex jig as shown
in Fig. 1A was used for providing reproducible
irradiation geometry, which was used for
calibration of center position of irradiation
volume of gamma chamber – 900. During
irradiation each dosimeter was provided with
a suffi cient build-up for achieving electronic
equilibrium [6] as shown in Fig. 1B.
Fig. 2 : Dosimetry tote box along with the
perspex sheets and boxes containing
mangoes
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33 0 C. Each dosimetry box was preceded and
followed by fi ve dummy boxes with the same
product.
3. RESULTS AND DISCUSSIONS
The dose values measured by optichromic
and alanine dosimeters for each position in
the dosimetry boxe are tabulated in Table 1.
Atomic Energy Act and Prevention of Food
Adulteration Act (PFA) Rules (Ministry of
Health and Family Welfare) 1994, 1998 and
2001, state that an agreement of in the range
of ± 10% (1σ) in dose verifi cation exercise
carried out with national standards laboratory
under actual irradiation conditions of food
irradiation plant, is acceptable. It is seen from
Table 1 that dose delivered to the mango is
consistent. Also, D min
and D max
values for both
the dosimetry systems agree very well.
Table 1: Dose verifi cation exercise between
M/s Innova and RSSD
4. REFERENCES
1. ISO/ASTM Standard 51261, Standard
guide for selection and calibration
of dosimetry systems for radiation
Processing, Annual Book of Standards,
American Society for Testing of Materials,
West Conshohocken, 2002, USA.
2. ASTM Standard E1026, Standard
practice for using Fricke referencestandard
dosimetry system, Annual
Book of Standards, American
Society for Testing of Materials, West
Conshohocken, 2003, USA.
3. ISO/ASTM Standard 51607, Standard
practice for use of the alanine-ESR
dosimetry system, Annual Book of
Standards, American Society for Testing
of Materials, West Conshohocken, 2002,
USA.
4. K. H. Chadwick, International doseassurance
in radiation technology,
International Atomic Energy Agency
Bulletin, 1982, 24(3), 21–27.
5. Indian Pharmacopoeia, Ultra violet and
visible spectrophotometry, 1996, India.
6. B. L. Gupta, S. R., Nilekani, and U.
Madhvanath, Dependence of dose on
dosimeter thickness under equilibrium
conditions, International Journal of
Applied Radiation and Isotopes, 1978,
29, 700–701.
The Maximum difference observed
between individual Innova and RSSD dose
value was 10%. The Over Dose Ratios of 2.46
measured by Innova agree well with the ODR
value 2.79 measured by RSSD.
As overall average dose delivered to the
food product agrees within ±10 % (1σ), the
Gamma Irradiation Facility viz., Innova was
issued license for mango irradiation by Atomic
Energy Regulatory Board.
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