Radiation Physics and Chemistry 71 (2004) 363–368

A study of the alanine dosimeter irradiation temperature

coefficient in the 77C to +50C range

Marc F. Desrosiersa,*, Sarene ́e L. Coopera, James M. Puhla, Anna L. McBainb,

Glenn W. Calvertb

a Ionizing Radiation Division, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA bClearant, Inc., 401 Professional Drive, Gaithersburg, MD 20879, USA

Abstract

Early researchers in alanine dosimetry established that, at least in the 0–50C temperature range, the amplitude

of the electron paramagnetic resonance signal of irradiated alanine grows linearly with irradiation temperature.

The irradiation temperature coefficient is derived from the slope of this response–irradiation temperature

relationship. However, very little data exists on the linearity of the response below 0C. Thus, the applicability

of the irradiation temperature coefficient determined above 0C to irradiations conducted below 0C is uncertain.

This work investigates the behavior of the alanine response irradiated using a Cobalt-60 gamma source

over the temperature range 77C to +50C. Since the temperature coefficient is known to be dose dependent

[Radiat. Phys. Chem. 57 (2000) 1], a series of dose response studies were conducted over a dose range of 0.5–100 kGy.

The study revealed that the temperature response deviates from linearity below 10C. The implications of these

observations to dosimetry will be discussed along with possible chemical mechanisms that would account for these

observations.

Published by Elsevier Ltd.

Keywords: Alanine; Dosimeters; Dosimetry; Irradiation; Radiation processing; Temperature; Temperature coefficient

1. Introduction

The temperature rise in materials treated with highintensity industrial ionizing radiation sources is an

important influence factor on the dosimeters used to

model or monitor the irradiation process. For alanine

dosimeters, the temperature during industrial irradiation

processing affects the radical production that is reflected

in the measured response of the dosimeter. The relationshipbetween the dosimeter’s radiation response to

absorbed dose and its temperature during irradiation is

termed its irradiation temperature coefficient. This

temperature coefficient is typically expressed in percent

change per degree, and for l-alanine is slightly above1

+0.1%/C for temperatures above zero Celsius (Nagy

et al., 2000).

In many industrial applications, temperature

control during irradiation is unnecessary. Processing

typically occurs at ambient temperatures, and dosimeters are characterized under the conditions of

use. Consequently, there has been little motivation

to characterize the dosimeter’s temperature response

characteristics in the negative temperature range.

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*Corresponding author. Tel.: +1-301-975-5639; fax: +1-

301-869-7682.

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

1The exact value is dependent on the formulation of the

alanine dosimeter.

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

doi:10.1016/j.radphyschem.2004.04.066
The advent of food irradiation expanded the range

of interest to sub-zero temperatures and extended the

range over which the coefficient was characterized to

approximately 10C. Recently, an ever-growing number of products are being irradiated that require

temperature control at even lower temperatures (e.g.,

sera used in cell cultures and tissue for implantation),

and these applications have fueled the need for

investigating the behavior of dosimeters at much lower

temperatures.

Many years ago, research in high-energy physics

stimulated a limited study on the alanine dosimeter

response characteristics at temperatures significantly

below zero Celsius. Coninckx et al. (1996) calibrated

alanine dosimeters irradiated at 4 and 77 K, temperatures not relevant to industrial applications. Moreover,

this work was performed on custom-made alanine

dosimeters. Response data for commercially produced

alanine dosimeters irradiated at low temperatures are

not available.

A key step in launching new irradiation applications

at low temperature will be regulatory approval of the

process. This requires measurements of absorbed dose

made by the industrial processor to be accurate and

traceable to a national metrology institute (NMI). As

alanine is the reference dosimetry system used by all

NMI’s in their high-dose calibration services, a characterization of the alanine low temperature dose

response was undertaken.

2. Experimental2

Commercially available alanine pellet dosimeters from

Harwell Dosimeters and Gamma Services were used in

these experiments. These dosimeters are similar in

dimension, mass and both use l-alanine, however, the

binders differ. Harwell uses a paraffin-based binder

while Gamma Service uses a non-paraffin (proprietary)

binder.

Irradiations were conducted in a Gammacell 220

(Nordion, Canada) Cobalt-60 gamma source with a

dose rate of about 18 kGy/h. A system that employed a

custom-designed aluminum holder combined with controlled-temperature airflow was used to achieve thermal

equilibrium from +50C to 55C during irradiation

(described in Nagy et al., 2000). For lower temperatures,

dry ice was the primary source of temperature control.

The dosimeters in the holder assembly were preequilibrated to the target temperature prior to irradiation and monitored throughout the irradiation to

maintain the target temperature within 1C throughout

the irradiation. The irradiation temperatures used to

represent the data in Figs. 1–5 were the computed

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0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

-20

Irradiation Temperature (°C)

Relative Response

-10 0 10 20 30 40 50 60

Fig. 1. Dose response relative to that at 25C for Harwell alanine dosimeters irradiated to absorbed doses of 5, 25, 50, 60, 80 and

100 kGy at fixed temperatures ranging from 10C to +50C.

2The mention of commercial products throughout this paper

does not imply recommendation or endorsement by the

National Institute of Standards and Technology, nor does it

imply that products identified are necessarily the best available

for this purpose.

364 M.F. Desrosiers et al. / Radiation Physics and Chemistry 71 (2004) 363–368
average of the temperatures measured for the duration

of the irradiation.

The electron paramagnetic resonance (EPR) signal

measurement protocol for ECS106 spectrometer measurements performed at NIST has been described in

detail previously (Nagy et al., 2000). The essential

features are that each dosimeter is measured at two

angles and that these measurements are normalized to

dosimeter mass and the spectrometer’s internal reference

material (ruby crystal). The average of the normalized

signals for both angles is used as the dosimeter response.

The EPR signal measurements by Clearant were

performed using a Bruker Biospin e-scan spectrometer

(microwave power, 0.517 mW; modulation amplitude,

0.101 mT). The signal of each dosimeter was measured,

and the signal normalized to the signal of an internal

EPR reference material (proprietary) signal to derive an

alanine-to-reference ratio. This ratio was further normalized to the dosimeter mass, and this mass-corrected

ratio was used as the dosimeter response.

The EPR dosimeter response for dosimeters irradiated

to 10C and higher underwent linear regression and

the resultant function was used to compute the EPR

response predicted at 25C. This value served as the

reference point from which the relative response for all

measurements was calculated. The value for the slope of

the relative response plotted versus irradiation temperature is the temperature coefficient. A third-order

polynomial was used to fit data that extended below

10C.

3. Results and discussion

Alanine dosimeters manufactured by Harwell were

irradiated to absorbed doses of 5, 25, 50, 60, 80 and

100 kGy at fixed temperatures ranging from 10C to

+50C. Analysis of the dosimeter relative response

(measured on a Bruker e-scan spectrometer) to irradiation temperature at each of the dose levels revealed no

significant difference in the derived temperature coefficient. With no discernable difference in the temperature

coefficient for each absorbed dose, the data from all

doses were combined and plotted as relative response

versus temperature (Fig. 1). A linear regression of these

data produced a temperature coefficient of +0.11%/C.

This value for the temperature coefficient is consistent

with previously reported values for l-alanine pellet

dosimeters (Nagy et al., 2000).

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0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

-90 -80 -70 -60 -50 -40 -30 -20 -10

Relative Response

5 kGy

25 kGy

50 kGy

60 kGy

80 kGy

100kGy

Temperature (°C)

0

Fig. 2. Dose response relative to that at 25C for Harwell alanine dosimeters irradiated to absorbed doses of 5, 25, 50, 60, 80 and

100 kGy at fixed temperatures ranging from 77C to 10C.

M.F. Desrosiers et al. / Radiation Physics and Chemistry 71 (2004) 363–368 365
Harwell alanine dosimeters were also irradiated to

these same doses but at temperatures in the range

77C to 10C. Dosimeter response measurements

were adjusted to the same relative value in Fig. 1 and

plotted versus irradiation temperature (Fig. 2). A

marked deviation from linearity was apparent at each

dose level. The deviation from linearity became greater

as the temperature decreased. The discrepancy in

relative response at 77C was 10–20% lower than

would be predicted from the linear trend in the high

temperature region (>10C) of the plot. Also, evident

from these data is a dependence of the relative response

curve on the absorbed dose. The relative response

decreases as the dose increases.

In order to correlate dosimeter measurements made

on the NIST and Clearant spectrometers, the relative

responses for each of the same sets of 5 and 50 kGy

dosimeters were measured on the two spectrometers.

The measurements for 50 kGy are shown in Fig. 3.

Clearly, there is excellent agreement between the data.

Similar agreement was observed for the 5 kGy data

comparisons (not shown).

The Bruker ECS106 spectrometer was used to study

temperature effects on lower doses. It is a more sensitive

instrument than the e-scan and can measure with greater

precision dosimeters irradiated to doses lower than

5 kGy. Harwell dosimeters irradiated to 0.5, 5, and

50 kGy over the same temperature range were measured

on this spectrometer. The dosimeter relative responses

measured for all three doses are compared in Fig. 4. As

in Fig. 2, the 50 kGy dosimeters showed a greater

temperature effect than the 5 kGy dosimeters. However,

at 0.5 kGy the trend is reversed.

Since commercial dosimeters are available from other

manufacturers, it was important to test whether these

observations are unique to the Harwell dosimeter. In

addition, binder influences on dosimeter temperature

effects have been postulated (Nagy et al., 2000). Gamma

Service alanine dosimeters were irradiated over a 55C

to +25C range for comparison to the Harwell

dosimeters previously characterized. Fig. 5 compares

the temperature effects on the dose response for both

dosimeters. The trends for each system are indistinguishable. The temperature coefficient measured for

Gamma Service dosimeters in the 15C to +25C

range was +0.10%/C.

4. Conclusion

The temperature coefficients measured for the two

most common commercial alanine pellet dosimeters in

the temperature range 10C to +50C are consistent

with published coefficients for other alanine pellet

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0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

-100 -80

Irradiation Temperature (°C)

Relative Response

50 kGy (NIST)

50 kGy (Clearant)

-60 -40 -20 0 20 40 60

Fig. 3. Dose response relative to that at 25C for Harwell alanine dosimeters irradiated to an absorbed dose of 50 kGy at fixed

temperatures ranging from 77C to +50C measured at NIST (ECS106 EPR spectrometer) and Clearant (e-scan EPR spectrometer).

366 M.F. Desrosiers et al. / Radiation Physics and Chemistry 71 (2004) 363–368
dosimeters (Nagy et al., 2000). Extension of the

temperature range to 77C revealed a dosimeter

response that is lower than that predicted from the

linear relationshipabove 10C. The deviation from

linearity increased with decreasing temperature and was

observed for both commercial dosimeters. Accurate

dosimetry requires a temperature-based correction of

the dosimeter response to that of the calibration

temperature. The observed deviations from linearity

are significant and corrections must be applied. Since the

alanine dose response over the range studied is nonlinear, these observed changes in response would

produce greater changes in the computed absorbed dose

(or lead to large errors without the appropriate

corrections). Subsequent investigations of the temperature effects on alanine film dosimeters as well as alanine

dosimeters from other manufacturers that use either dlalanine or l-alanine crystals exhibit the same effect

(Desrosiers and Cooper, unpublished results).

The magnitude of the temperature effect on dosimeter

response increased with dose from 5 to 50 kGy (Fig. 2).

However, a general recommendation regarding the

dose–response relationship as a function of temperature

cannot be made since potentially conflicting results were

observed in Fig. 4; here, the 0.5 kGy response curve lies

between that of 5 and 50 kGy. A dose effect on the

temperature coefficient for alanine pellets (from different manufacturers) was previously observed for temperatures above zero (Nagy et al., 2000). However, the

dose effect reported by Nagy was not observed for the

Harwell and Gamma Service pellets measured here. The

dose-dependent effect at low temperature should be

examined in greater detail.

The mechanism by which the irradiation temperature

affects the alanine dosimeter response to absorbed dose

cannot be derived directly from these data. However, a

study of what is currently known about the solid state

radiation chemistry of crystalline alanine offers a

plausible explanation for these observations. Desrosiers

et al. (1995) published the first direct evidence for

detectable levels of chemically distinct secondary alanine-derived radicals in irradiated alanine crystals.

Subsequent spectroscopic analysis studies produced

postulated structures and relative yields of these radicals

(Malinen et al., 2003, and references therein). The

implications of these studies are that the observed

alanine spectrum is actually a composite of two or more

alanine-derived radicals. The observed changes in the

measured alanine signal amplitude likely result from the

effect of temperature on the reaction rates for the

various pathways available to the reactive chemical

intermediates. The net effect of this influence is an

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-35%

-30%

-25%

-20%

-15%

-10%

-5%

0%

5%

10%

% Difference in response relative to 25°C

0.5 kGy Response

5 kGy Response

50 kGy Response

Poly. (0.5 kGy Response)

Poly. (5 kGy Response)

Poly. (50 kGy Response)

-80

Irradiation Temperature (°C)

-60 -40 -20 0 20 40 60

Fig. 4. Relative dose response of Harwell alanine dosimeters irradiated to absorbed doses of 0.5, 5 and 50 kGy at fixed temperatures

ranging from 77C to 10C (measured at NIST).

M.F. Desrosiers et al. / Radiation Physics and Chemistry 71 (2004) 363–368 367
alteration in the relative distributions of the alaninederived radical species that combine to produce the

signal amplitude measured for dosimetry.

The implications for dosimetry are that linear

temperature corrections can only be applied in the

10C to +50C irradiation temperature range. A nonlinear correction must be applied below 10C. For the

NIST measurements between 0.5 and 50 kGy a thirdorder polynomial3 would be suggested to apply response

corrections. However, it has yet to be determined if this

correction is dosimeter batch specific. These data also

suggest that, depending on the precision requirements of

the dosimetry system employed, a single relationship for

this correction may be applied below 50 kGy. However,

it is likely that corrections related to absorbed dose

should be used above 50 kGy.

References

Coninckx, F., Janett, A., Kojima, T., Onori, S., Pantaloni, M.,

Schoenbacher, H., Tavlet, M., Wieser, A., 1996. Responses

of alanine dosimeters to irradiations at cryogenic temperatures. Appl. Radiat. Isot. 47, 1223–1229.

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.

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., Puhl, J.M., Desrosiers, M.F., 2000. Advancements in

the accuracy of the alanine dosimetry system. Part 2. The

influence of the irradiation temperature. Radiat. Phys.

Chem. 57, 1–9.

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-35%

-30%

-25%

-20%

-15%

-10%

-5%

0%

5%

10%

% Difference in response relative to 25 °C

Harwell (eScan)

Gamma Service (eScan)

-80

Irradiation Temperature (°C)

-60 -40 -20 0 20 40 60

Fig. 5. Relative dose response of Harwell alanine dosimeters irradiated to absorbed doses of 50 kGy at fixed temperatures ranging

from 77C to +50C (Harwell) and 55C to +25C (Gamma Service).

3

A least-squares fit to the data (collectively) in Fig. 4 gave the

relationship, R = (1.3E5)T3 + (1.2E3)T2 + (1.5E1)T0.26,

where R is the dosimeter response (in percent relative to 25C) and

T is the irradiation temperature (C). For this experiment, the

difference in the computed percent response at the irradiation

temperature to the percent response at 25C (the calibration

temperature) will give the percent correction to apply to the

measured response of the irradiated dosimeter (only if the

irradiation temperature is below 10C). For dosimeters

irradiated above 10C, the temperature effect is linear and the

temperature coefficient is used to correct dosimeter response to the

calibration temperature.

368 M.F. Desrosiers et al. / Radiation Physics and Chemistry 71 (2004) 363–368