Absorbed-dose/dose-rate dependence studies for the alanine-EPR

dosimetry system

M.F. Desrosiers , J.M. Puhl

Ionizing Radiation Division, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, USA

article info

Keywords:

Alanine

Dosimeter

Dosimetry

Electron beam

Electron paramagnetic resonance

Gamma-ray

ionizing radiation

irradiation temperature

abstract

Over the course of the last decade, routine monitoring of the alanine dosimetry system revealed a small

but significant observation that, after examination, led to the characterization of a previously unknown

absorbed-dose-dependent, dose-rate effect for the alanine system. The newly discovered rate effect is of

potential concern for electron-beam dosimetry, since electron-beam dosimetry typically derives its

traceability to national standards through comparisons to gamma-ray calibrations of the dosimetry

system. The largest discrepancy in source dose rates is between gamma-ray sources and electron-beam

accelerators. Investigating the influence of temperature on the alanine rate effect is an important first

step in preparation for a comparison study between electron-beam and gamma-ray dosimetry. Here,

new data is presented on the influence of irradiation temperature (from 40 to +50 1C) on the dose-rate

effect measured at 50 kGy.

Published by Elsevier Ltd.

1. Introduction

Check standards are used by the NIST Ionizing Radiation

Division to monitor the performance of the alanine dosimetry

system that is central to its high-dose transfer dosimetry service.

These measurements are performed to confirm the operational

readiness of the calibration curve. Deviations from the expected

check-standard values can result from a wide range of sources that

include both manufacturing abnormalities in a dosimeter and

spectrometer-related changes. Recently, check-standard measurement deviations unveiled a previously unknown rate effect for the

alanine dosimetry system (Desrosiers et al., 2008). This rate-effect

study characterized a complex relation between the radiation

chemistry of crystalline alanine and the applied dose rate that was

also dependent on the absorbed dose. That the rate effect only

becomes significant above 5 kGy likely contributed to it only

recently being discovered despite decades of research in alanine

dosimetry. It was learned that the effect is intrinsic to alanine and

is not dependent on the chemical form or manufacturing

formulation of the alanine dosimeter. The study postulated that

the production of one (or more) of the radiation-induced alanine

radicals is dependent on the dose rate.

Typically, high-dose-rate calibration sources maintained by

National Measurement Institutes (NMIs) are calibrated against

low-dose-rate sources that are used to realize the gray with a

primary standard (e.g., calorimeter). If a dose rate is established

by comparison of dosimeters irradiated in a high-rate gamma

calibration source to dosimeters calibrated in a low-rate source at

absorbed doses in the kGy range, errors could be introduced in the

dose rate for the high-rate calibration source. If this calibration

source is in turn used for service work, a dose-dependent error

would be further transferred to industrial customers. Even if

the scenario cited above is not relevant for a specific NMI,

two additional considerations remain. First, caution would be

advised to avoid using alanine dosimeters at high doses (45 kGy)

for transfer calibrations of low-dose-rate (o2 Gy s1

) sources;

second, to avoid using a gamma source with a low-dose-rate

(o 2 Gy s1

) to calibrate alanine dosimeters.

The range of dose rates used in the previous study (0.1–5 Gy

s1

, Desrosiers et al., 2008) is typical for gamma-ray sources.

However, electron-beam irradiators are being used for many

industrial applications. This raises the question: do rate effects

exist for the much higher dose rates (104 Gy s1 and higher)

employed in electron-beam dosimetry? The electron and gamma

irradiation technologies are so vastly different that a subtle rate

effect could be masked by larger effects (e.g., irradiation

temperature). Electron-beam irradiations are typically accompanied by significant increases in temperature, whereas gamma-ray

irradiations used to calibrate the dosimetry system are typically

temperature-controlled at ambient or moderate temperatures.

This work presents the results of measurements that are

fundamental to the electron-beam dosimetry investigation.

Previously published 1995-through-2007 source-comparison

measurements (Desrosiers et al., 2008) compared the response

ratios of dosimeters irradiated (to the same dose) with gamma

sources of dose rates lower than 1.4 Gy s1 (these include the Pool,

ARTICLE IN PRESS

Contents lists available at ScienceDirect

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

Radiation Physics and Chemistry

0969-806X/$ - see front matter Published by Elsevier Ltd.

doi:10.1016/j.radphyschem.2009.03.025

Corresponding author. Tel.: +1301975 5639; fax: +1301869 7682.

E-mail address: marc.desrosiers@nist.gov (M.F. Desrosiers).

Radiation Physics and Chemistry 78 (2009) 461–463
GC45, and GC232) to dosimeters irradiated concurrently with

gamma sources of dose rates greater than 3.5 Gy s1

. The absorbed

doses for these measurements were grouped as low dose (1 kGy)

or high dose (predominately 25 kGy, but one 20 kGy and one

30 kGy ratio is included). The 1 kGy ratios of low-rate to high-rate

measurements remain clustered about unity consistently from

1995 through 2007 regardless of the gamma source used. The

20–30 kGy measurement ratios consistently indicate a response

1–2% lower than that from the 1 kGy measurements regardless of

the gamma source used. A linear regression of previously

published data (Desrosiers et al., 2008) for the 20–30 kGy ratio

data suggests that maximum effect (computed at zero dose rate)

should be approximately 2%. The regression intercepts unity at a

dose rate of approximately 4 Gy s1

. The regression of the

20–30 kGy ratio data becomes significant (intercepts 0.99 of the

y-axis) at a dose rate of approximately 2 Gy s1

. Measurements to

date suggest that the effect is not linear with absorbed dose rate,

so the linear approximations would represent conservative

estimates. A more comprehensive study of high- and low-dose

dosimeter response ratios between 1 and 4 Gy s1 is needed to

assess the dose-rate effect transition. In the interim, these data

will guide the assignment of uncertainties to this effect for

calibrations of sources in this dose-rate range.

Alanine as a dosimeter for the high dose rates produced by

high-energy industrial electron-beam accelerators is worthy of

investigation, because the dosimetry system is typically calibrated

by comparison to much lower dose-rate gamma-ray sources. The

differences between electron-beam and gamma-ray irradiation

processing will present several challenges to the investigation.

One of these challenges, the influence of irradiation temperature,

can be controlled for gamma-ray dosimetry in a laboratory facility

but is uncontrolled for high-dose electron-beam irradiations. In

practice, the dosimeter response is corrected for differences

between the average irradiation temperature of the field dosimeter and the irradiation temperature used for the dosimetry

system calibration. This dosimeter response adjustment is made

using a previously determined temperature coefficient. However,

applying these corrections in studies designed to search for a

dose-rate effect between electron-beam and gamma-ray irradiators without first investigating the influence of temperature on

the rate effect itself may have unintended consequences. If, as

postulated by Desrosiers et al. (2008), the rate effect is a result of

changes in the free radical distribution, then the irradiation

temperature could potentially further influence the reaction rates

that govern the yield of the different free radicals. In turn, the

altered free radical distribution could (positively or negatively)

alter the net electron paramagnetic resonance (EPR) spectral

amplitude that manifests itself as the observed dose-rate effect.

Here, the term net spectral amplitude is used to convey the concept

that the EPR amplitude used as a measure of the dosimeter

response is actually a composite of EPR amplitudes from at least

two, likely three, and possibly more alanine free radicals (Wieser

et al., 1993; Desrosiers et al., 1995; Malinen et al., 2003).

2. Experimental

The absorbed doses for this study were delivered with either of

two Gammacell 220 60Co gamma-ray sources (Nordion, Canada).1

The Gammacell activities were: 108 TBq (2.9 kCi, serial number

GC232); and 531 TBq (14 kCi, serial number GC207), all as of

August 1, 2008. Alanine dosimeters were irradiated in the center

position geometry of each source. The irradiation geometry and its

calibration are described in NIST SP250-45 (Humphreys et al.,

1998) with the exception of a modification to the calibration

scheme described by Desrosiers et al. (2008). Temperature during

the irradiation was controlled by using a high-flow air shower

from a TurboJet (FTS Systems) and monitored with a type-T

thermocouple.

The dosimeters were measured by electron paramagnetic

resonance spectrometry. A Bruker Biospin ECS106 EPR spectrometer was configured to specifically measure alanine pellet

dosimeters (Humphreys et al., 1998). The EPR amplitude is a ratio

of the alanine-EPR amplitude of the center resonance and the ruby

reference material EPR amplitude (as described in Nagy et al.,

2000). The alanine/ruby EPR amplitude ratio is normalized by

dividing the ratio value by the mass of the individual dosimeter.

The resultant value is referred to as the dosimeter response. The

alanine-EPR recording parameters common to all measurements

in this study were: frequency, 9.684 GHz; center field, 345.5 mT;

magnetic-field sweep width, 1.0 mT; modulation amplitude,

0.285 mT; time constant, 1.3 s; and sweep time, 21 s.

Alanine dosimeter response ratios were determined from

dosimeters irradiated in tandem in the GC207 and GC232

Gammacell 220 60Co irradiators. The tandem irradiations were

timed such that they would terminate at approximately the same

time and permit the post-irradiation readout times to be

equivalent. Dosimeters were stored under ambient laboratory

conditions for 24 h before measurement.

3. Results and discussion

A set of experiments was undertaken to examine the effect of

irradiation temperature on the dose-dependent dose-rate effect

previously observed (Desrosiers et al., 2008). Response ratios for

dosimeters irradiated at high dose (50 kGy) and low dose (1 kGy)

were determined by comparing dosimeter responses of equivalent

dose that were achieved with different dose rates. The Gammacells GC207 (3.1 Gy s1

) and GC232 (0.62 Gy s1) were used for

these irradiations. At each dose level in each Gammacell, an

average irradiation temperature was maintained at each of the

following: 40, 10, 0, +10, +24, +38, or +50 1C. The dosimeter

response at each dose level and temperature was used to compute

ARTICLE IN PRESS

0.97

0.98

0.99

1

1.01

1.02

1.03

-50

Irradiation Temperature (Celsius)

Response Ratio (LowRate / High Rate) 

1 kGy

50 kGy

-40 -30 -20 -10 0 10 20 30 40 50 60

Fig. 1. Plot of the alanine dosimeter response ratio (low rate, 0.62 Gy s1, over high

rate, 3.1 Gy s1

) versus the irradiation temperature in degrees Celsius for 1 kGy

(circles) and 50 kGy (diamonds). The error bars represent the standard uncertainty

(k 1⁄4 1).

1 Certain commercial equipment, instruments, or materials are identified in

this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor

does it imply that the materials or equipment identified are necessarily the best

available for the purpose.

462 M.F. Desrosiers, J.M. Puhl / Radiation Physics and Chemistry 78 (2009) 461–463
response ratios in terms of the dosimeter response for the lowdose-rate source divided by the dosimeter response for the highdose-rate source. These data are summarized in Fig. 1. The

nonequivalence of the response ratios between 1 and 50 kGy at

+24 1C was comparable to the previous findings (Desrosiers et al.,

2008). The results obtained for the higher temperatures, +38 and

+50 1C, though within the range of the +24 1C data, suggested that

there may be a trend towards a weaker dose-rate effect at elevated

temperatures. The dose-rate effects at 0 and +10 1C are consistent

with the +24 1C data. However, a distinct change in the dose-rate

effect occurred at the irradiation temperatures of 10 and 40 1C.

There was no measurable rate effect at 50 kGy at either

temperature.

4. Conclusions

The initial concern that the elevated temperatures common to

high-dose electron-beam irradiations would negatively impact

the ability to explore the possibility of dose-rate effects under

these conditions has been alleviated. The data suggest that there

may be a trend towards a reduction in the rate effect with

increasing irradiation temperature. This is a positive result as the

uncertainty in the electron-beam irradiation temperature would

not negatively influence the precision of the alanine dose

assessment. The finding that there is no dose-rate effect for

irradiation temperatures of 10 and 40 1C is of particular

interest to dosimetry for electron-beam processing of food and

medical products. Measurements to determine the rate effect

temperature profiles for absorbed doses between 5 and 100 kGy

are planned.

References

Desrosiers, M.F., Burlinska, G., Kuppusamy, P., Zweier, J., Yaczko, D.M., Auteri, F.P.,

McClelland, M.R., Dick, C.E., McLaughlin, W.L., 1995. Research and development

activities in electron paramagnetic resonance dosimetry. Radiat. Phys. Chem.

46, 1181–1184.

Desrosiers, M.F., Puhl, J.M., Cooper, S.L., 2008. An absorbed-dose/dose-rate

dependence for the alanine-EPR dosimetry system and its implications in

high-dose ionizing radiation metrology. NIST J. Res. 113 (2), 79–95.

Humphreys, J.C., Puhl, J.M., Seltzer, S.M., McLaughlin, W.L., Desrosiers, M.F., Nagy,

V.Y., Bensen, D.L., Walker, M.L., 1998. Radiation Processing Dosimetry

Calibration Services: Manual of Calibration Procedures. NIST Special Publication 250–45, March.

Malinen, E., Heydari, M.Z., Sagstuen, E., Hole, E.O., 2003. Alanine radicals, Part 3.

properties of the components contributing to the EPR spectrum of X-irradiated

alanine dosimeters. Radiat. Res. 159, 23–32.

Nagy, V.Yu., Sleptchonok, O.F., Desrosiers, M.F., Weber, R.T., Heiss, A.H., 2000.

Advancements in accuracy of the alanine dosimetry system, Part 3. usefulness

of an adjacent reference sample. Radiat. Phys. Chem. 59, 429–441.

Wieser, A., Lettau, C., Fill, U., Regulla, D.F., 1993. The influence of non-radiation

induced ESR background signal from paraffin-alanine probes for dosimetry in

the radiotherapy dose range. Appl. Radiat. Isot. 44, 59–65.

ARTICLE IN PRESS

M.F. Desrosiers, J.M. Puhl / Radiation Physics and Chemistry 78 (2009) 461–463 463