1. Introduction

Alanine-based ionizing radiation dosimetry is firmly

woven into the fabric of high-dose radiation metrology.

Because of its superior attributes, alanine dosimetry

was recognized at an early stage to be of great importance to National Measurement Institute (NMI) services and programs. After many years of use at the NMI

level, confidence in the system has grown such that alanine dosimetry use is growing in industry. The transfer

of this technology has enabled industrial users of radiation to gain greater control over radiation applications

for food/materials processing and medical-device sterilization. When used as a routine dosimeter, alanine

dosimetry has proven to be an accurate and robust system, ideal for the often harsh conditions of industrial

radiation processing. It is relatively insensitive to a

variety of common environmental influences such as

humidity, ambient light and temperature. This improves

system reliability and reduces laborious/costly troubleshooting events related to other systems commonly

used (e.g., radiochromic dosimetry).

From the NMI’s perspective, the broad absorbeddose range of the alanine system and reported [1] energy and dose-rate independence offered great versatility

for calibrating a wide variety of customer-based radiation sources. The National Institute of Standards and

Technology (NIST) began developing its alanine system in the early 1990s to replace radiochromic-dye film

dosimeters. This work led to the formal establishment

of the alanine dosimetry calibration service approximately ten years ago. Over the course of the last

decade, routine monitoring of the 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. Though the potential impact of

this effect is anticipated to be limited for NIST’s customer-based transfer dosimetry service, much greater

Volume 113, Number 2, March-April 2008

Journal of Research of the National Institute of Standards and Technology

79

[J. Res. Natl. Inst. Stand. Technol. 113, 79-95 (2008)]

An Absorbed-Dose/Dose-Rate Dependence for

the Alanine-EPR Dosimetry System and Its

Implications in High-Dose Ionizing

Radiation Metrology

Volume 113 Number 2 March-April 2008

M. F. Desrosiers, J. M. Puhl,

and S. L. Cooper

National Institute of Standards

and Technology,

Gaithersburg, MD 20899

marc.desrosiers@nist.gov

james.puhl@nist.gov

sarenee.cooper@nist.gov

NIST developed the alanine dosimetry system in the early 1990s to replace

radiochromic dye film dosimeters. Later in

the decade the alanine system was firmly

established as a transfer service for highdose radiation dosimetry and an integral

part of the internal calibration scheme supporting these services. Over the course of

the last decade, routine monitoring of the

system revealed a small but significant

observation that, after examination, led to

the characterization of a previously

unknown absorbed-dose-dependent, doserate effect for the alanine system. Though

the potential impact of this effect is anticipated to be extremely limited for NIST’s

customer-based transfer dosimetry service,

much greater implications may be realized

for international measurement comparisons between National Measurement

Institutes.

Key words: alanine; dosimeter; dosimetry; electron paramagnetic resonance;

gamma ray; ionizing radiation.

Accepted: April 14, 2008

Available online: http://www.nist.gov/jres
implications may be realized for international measurement comparisons between NMI’s.

2. Calibration of NIST Gamma Sources

2.1 Gamma Sources for High-Dose Calibration

Services

For industrial high-dose dosimetry calibration services, the measurement quantity of interest is absorbed

dose (to water, primarily), reported in units of gray. At

NIST, absorbed dose is realized by a water calorimeter

in a gamma-ray field produced by a Vertical Beam 60Co

source [2] with an activity (as of April, 2007) of 48 TBq

(1.3 kCi). The technical specifications for this source

were described previously [3]. Because the dose rate

for this source is relatively low, a high-dose-rate source

is needed to perform customer calibrations. The bulk of

the services are provided through three Gammacell 220

60Co irradiators (Nordion, Canada).1 Their activities

are: 37 TBq (1.0 kCi, serial number GC45); 137 TBq

(3.7 kCi, serial number GC232); and 666 TBq (18 kCi,

serial number GC207), all as of April 2007. Though

also available for service operation, the Pool 60Co

source (0.15 kCi as of April 2007) is rarely requested

for calibration work by customers because its low

absorbed-dose rate is no longer relevant to industrial

needs; however, the Pool source continues to play an

important role in the NIST source calibration scheme

described here (See 2.3).

2.2 NIST Alanine Dosimetry

The alanine dosimetry system has offered great benefits and flexibility to industrial dosimetry and the supporting calibration services. Alanine has a dose range

that spans most industrial applications [4]. The alanine

system is energy independent (above 100 keV) and,

prior to this work, there have been no reports of doserate effects. Radiation quality issues are not significant

for electrons and photons; recent claims of an electronphoton dose discrepancy [5] lie within the uncertainty

of the high-dose certification service. Irradiation temperature and post-irradiation temporal effects have

been extensively studied and are minimal [6-8].

Relative humidity effects on the alanine-Electron

Paramagnetic Resonance (EPR) measurement [9] are

compensated for through the use of an internal EPR reference material [10]. These attributes, together with a

rugged commercially manufactured dosimeter and a

high-accuracy/precision EPR spectrometer system,

enable NIST to operate a postal-based transfer dosimetry system with an expanded uncertainty of less than

2 % (coverage factor, k = 2).

The alanine pellet dosimeters currently used in the

NIST calibration services are manufactured by Gamma

Services (Germany) and distributed through Far West

Technology (Goleta, CA). These dosimeters have been

used by NIST since the inception of the alanine-based

services. The current transfer dosimetry service protocol for the alanine absorbed dose measurement is found

in Section 4 of Procedure 12 in the Ionizing Radiation

Division (IRD) Quality Manual [11]. For the internal

NIST source dose-rate calibrations, the absorbed dose

is not computed. As described in the protocol [11], the

ratio of the alanine EPR signal amplitude to the ruby

reference EPR signal amplitude is normalized to

dosimeter mass and averaged for two measurement

angles; these dosimeter values, referred to as the

dosimeter response, are used to calibrate the dose rate

for a fixed irradiation geometry for each radiation

source (see Sec. 2.3).

2.3 Gamma Source Calibrations

Calibration of the gamma sources within the NIST

high-dose calibration facility are performed by measuring the ratio of the alanine dosimeter response for the

source being calibrated to that of a reference source.

The absorbed dose for these internal calibrations is 1

kGy or less.2 This approach simplifies the source comparison to a measurement of two quantities, dosimeter

response and time. Absorbed dose is not computed for

this calibration exercise because these added steps will

introduce additional uncertainties inherent in the calibration curve, and it avoids any issues that might arise

from non-linearity in the dosimetry system dose

response. Moreover, the very fact that this responseper- time calibration scheme was able to reveal the subtle rate-dependence described here is strong support for

the validity of the method.

The four 60Co sources described in Sec. 2.1 are used

for NIST high-dose calibration services. Prior to 2004,

the Pool source and the Gammacells were each caliVolume 113, Number 2, March-April 2008

Journal of Research of the National Institute of Standards and Technology

80

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.

2 The alanine EPR signal amplitude is linear with absorbed dose up

to ≈4 kGy.
brated by a direct dosimeter response ratio to the

Vertical Beam 60Co source [3]. However, the Vertical

Beam 60Co source dose rate has decayed to a level that

requires excessive periods of time (>24 h) to perform

comparisons at the absorbed doses (≥1 kGy) routinely

used. Since the Vertical Beam 60Co source irradiations

are performed under water, with the water surface in

the vessel exposed to the room environment, there were

concerns that a variation in the water level would contribute significantly to the uncertainty of the measurement, as it would be difficult to keep the water level

constant for a prolonged period. To address the increasingly longer Vertical Beam irradiation times, modifications to the calibration scheme were developed. In 2004

the calibration scheme was modified so that the Vertical

Beam 60Co source would be compared only to the Pool

source. To improve several aspects of the measurement,

the absorbed dose for this comparison was adjusted

lower (140 Gy) for the Pool/Vertical-Beam source comparison. The three Gammacells are calibrated by comparison to the Pool source. The Pool source serves well

as an intermediary source in the calibration scheme as

its dose rate is closer to that of the Gammacells; this

permits longer exposure times, resulting in reduced

timer uncertainties.

The calibration scheme begins with the known dose

rate of the Vertical Beam 60Co source, established in

1990 [2]. To transfer that dose rate, eight alanine pellets

are irradiated in the calorimeter water tank. The pellets

are stacked in a watertight polystyrene cylinder whose

axis is fixed perpendicular to the Vertical Beam 60Co

source at a scale distance of 58.8 cm. The water surface

is set at a scale distance of 53.8 cm. This design differs

from the published scheme [3]. In the published

scheme, this irradiation was done in a polystyrene

phantom and a scaling theorem was used to correct for

differences in photon interaction cross sections

between polystyrene and water. This direct underwater

measurement was an improvement as it eliminated

scaling theorem uncertainties. In the current calibration

scheme the dosimeters are irradiated at the appropriate

distance underwater, and no additional corrections are

applied to the measured data.

Concurrent to the Vertical Beam irradiation, alanine

dosimeters are irradiated to the same absorbed dose

(described in [13]) in the absolute center of the isodose

region of the Pool-source gamma field. This comparison may be repeated as necessary to achieve an acceptable precision of 0.5 %. The dosimeters are measured

using EPR, and the dosimeter response is divided by

the irradiation time to convert to units of response•s–1.

Once the measurements are converted to these common

units, the established dose rate in the Vertical Beam

source [2] can be used to determine the dose rate in the

Pool source.

Similarly, a series of comparisons are made between

the dose rates at the center positions of the Pool source

and the three Gammacells (GC45, GC232, and GC207)

with the alanine transfer vial placed on a polystyrene

pedestal set to position the dosimeters in the absolute

center of the isodose region of the gamma field. For

these comparisons a higher dose is used (e.g., 1 kGy) to

reduce the contribution of uncertainty in the timer settings for the highest dose-rate Gammacell. In the

GC232 and GC207, irradiations are performed on a

pedestal either in a stainless-steel dewar or without a

dewar; the dewar is used to improve temperature control at the extremes of the irradiation temperature

ranges. The dosimeters are measured and the

response•s–1 is determined. The center-position dose

rate for each Gammacell is determined by comparison

to the Pool source center-position dose rate.

It should be noted that the equivalent transit time, the

time subtracted from the timer setting that accounts for

the absorbed dose received by the dosimeters during

the delivery of the dosimeters to and from the irradiation position, is determined for each source. To measure the equivalent transit time, alanine dosimeters are

irradiated for a series of very short times. Typically,

these times are 5 s, 10 s, 20 s, 30 s, 40 s, and 50 s. The

dosimeter response is measured and plotted versus irradiation time. A linear regression of these data is extrapolated to the x axis. The absolute value of the x intercept is the equivalent transit time.

Customer-supplied dosimeters for calibration are

routinely irradiated in one of three calibration geometries: ampoule, Perspex, and film block. The rates for

each of these positions in the Gammacells (though not

all positions are used in each Gammacell), with and

without a dewar present, are determined by comparison

of the response•s–1 for dosimeters irradiated in these

positions to the response•s–1 for dosimeters irradiated in

the center position of the respective Gammacell. This

final portion of the calibration scheme remains

unchanged from that previously published [3]. As a

final check of the dose rates, all irradiation geometries

are compared to confirm an equivalent measurement

response for dosimeters irradiated to 1 kGy.

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2.4 Calibration Service Maintenance

2.4.1 Calibration-Source Dose Rates

Approximately annually, dosimeter-response comparisons between the Gammacells, Pool and Vertical

Beam 60Co sources are performed. The ratios of source

dose-rates are determined and ongoing control charts

are maintained. These measurements have a standard

uncertainty of approximately 0.5 % (k = 1).

When possible, dosimetry comparisons are performed between NIST and the high-dose calibration

facility of the National Physical Laboratory of the

United Kingdom. Dosimeters from each facility are

exchanged, measured, and the results compared.

Participation in other NMI or multi-NMI international

comparisons occurs as appropriate [13].

2.4.2 Transfer Service Dosimetry System

Calibration

The alanine dosimetry system is calibrated approximately annually or whenever the EPR measurement

configuration changes and/or a new dosimeter lot is

used. The details of the calibration procedure are

described in the IRD Quality Manual [11].

The calibration curve for the dosimetry system is

maintained through the use of check standards. The

check standards are alanine dosimeters that have been

irradiated to each of the following doses: 1 kGy,

10 kGy, and 50 kGy. These check standards are routinely measured within 48 hours after irradiation, as well as

prior to transfer dosimetry service measurements. After

the initial calibration of the dosimetry system, these

dosimeters are measured periodically (on the average,

monthly). Data from these check standards are compiled into a control chart for tracking and comparison.

Check standards for doses outside of the 1 kGy–50 kGy

range are generated and measured as needed.

2.4.3 Calibration Curve History

In August 2004 the alanine dosimetry system was

calibrated using the GC232 gamma source. Shortly

thereafter the GC207 gamma source and all its irradiation geometries were calibrated (see Sec. 2.3). These

calibrations confirmed the equivalence of response for

dosimeters irradiated to 1 kGy in all three NIST

Gammacells (GC207, GC232, GC45). For the remainder of 2004 through March 2005, check standards were

created using the GC207 because its high dose rate

reduced the irradiation time necessary for this work.

3. The Alanine Dosimeter Dose/DoseRate Effect

3.1 Observation of a Dose-Rate Effect

As described in Sec. 2.4, check standards are used to

monitor the performance of the dosimetry system used

for transfer dosimetry. The measurements are performed to confirm the proper operation of the spectrometer and the reproducibility of the calibration

curve. Deviations from the expected values can result

from abnormalities in a dosimeter or calibration errors

made with the reference radiation source.

A check-standard measurement deviation was noted

during a scheduled measurement session in April 2005.

After months of check-standard measurements that

produced consistent results, a distinct change in the

measurement trend was observed on April 1, 2005. This

change coincided with a change in the calibration

source used to create the check standard. Because

check standards are used at relatively high doses, the

highest rate source (GC207) was routinely used. On

this date, however, the GC232 was used to create check

standards because the GC207 was occupied with scheduled irradiations. Because the dose rates in all source

geometries are calibrated and found to produce equivalent dosimeter responses at 1 kGy, the sources were

considered interchangeable.

Figure 1 shows a record of the check-standard measurements from late 2004 through April 2005. The relative difference between the absorbed dose determined

from the calibration curve and the dosimeter’s absorbed

dose is plotted against the date of measurement. In

April 2005, a dose-dependent inversion of the data

trend is observed. The 50 kGy measurements that were

consistently reading higher than the other doses were

now the lowest, and although the 10 kGy doses were

lower as well, the decrease was not as dramatic.

Curiously, the 1 kGy doses were consistent with the

previously measured doses in the chart. This observation initiated a study of the cause for the change in

measurement results. Over the course of the month of

April the irradiation conditions and records were

checked, and the check-standard measurements were

repeated several times using the GC232 as the irradiation source (Fig. 1). Concurrent with these measurements, several investigations were conducted to search

for a source of error in the irradiation process. Timers

(redundant timers are used on each Gammacell) were

checked and found to be in working order. The mechanics of the GC232 sample-chamber movement were

examined to confirm that the position of the chamber

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82
was reproducible at the start of the irradiation, did not

move during the course of the irradiation, and did not

move from the irradiation position prematurely. Checks

of the irradiation geometry included an extensive

examination of the dependence of the measurements on

the position of dosimeters placed within the sample

chamber. No timer or mechanical errors were detected,

and the sample-irradiation geometry was confirmed to

be correct. The temperature-control measurement and

monitoring system for the GC232 and GC207 were also

checked and found to be in proper working order.

As a next step in the root-cause analysis, a calibration curve was created in the GC232 (1 kGy, 2 kGy,

3 kGy, 5 kGy, 7 kGy, 10 kGy, 20 kGy, 30 kGy, 50 kGy)

and the GC207 (1 kGy, 2 kGy, 5 kGy, 10 kGy, 30 kGy,

50 kGy). Four dosimeters were irradiated at each dose.

All sets of calibrated dosimeters were measured by

EPR, and a least-squares fit was applied to the data.

The function used had the same mathematical form for

each curve, which was also the same as the curve established previously in August 2004. A comparison of

these two calibration curves is shown in Fig. 2. The differences between the two calibrations are best measured by the relative difference between the measured

absorbed doses and (as computed by the source dose

rate) the doses delivered (Fig. 3). The open circles represent the 2005 GC207 calibrated dosimeter doses

computed from the 2005 GC207 calibration function;

these represent the relative residuals (in percent) of the

2005 GC207 calibration curve and are shown for the

purpose of comparison. The open triangles in Fig. 3

represent the 2005 GC207 calibrated dosimeter doses

computed from the 2004 GC232 calibration function. A

large dose-dependent trend is observed in the filled circles that represent the 2005 GC232 calibrated dosimeter doses computed from the 2005 GC207 calibration

function. At doses below 10 kGy the GC232 dosimeter

doses are mostly consistent with the GC207 dosimeter

doses computed from the same (2005) calibration function. However, a significant deviation of the GC232

dosimeter doses from the GC207 dosimeter doses is

evident above 20 kGy, and this discrepancy increases as

the dose increases to 30 kGy and 50 kGy. The data represented by filled triangles in Fig. 3 represent the

GC232 dosimeter doses computed from the 2004

GC232 calibration curve; these data follow the trend of

the GC232 dosimeter dose measurements from the

2005 GC207 calibration curve.

The dosimeter responses were compared directly to

verify that the differences between the dosimeter sets

were not an aberration caused by the curvature of the

dosimeter response in the high-dose saturation range.

In Fig. 4 the ratio of the GC232/GC207 dosimeter

responses are plotted versus absorbed dose. Clearly, the

effect observed in Fig. 3 remains apparent, though the

magnitude of the effect is reduced. The difference

between these data sets can be attributed to the doseVolume 113, Number 2, March-April 2008

Journal of Research of the National Institute of Standards and Technology

83

Fig. 1. A quality-control check-standard plot for the high-dose transfer dosimetry service. The relative difference of the computed absorbed dose for the check-standard measurement from the absorbed dose delivered is

plotted versus the measurement date. Three doses were measured on each date: 1 kGy, 10 kGy, and 50 kGy (the

symbols for each are defined in the legend). Dosimeters that measured ± 1 % from the target dose are deemed

acceptable.
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Fig. 2. Calibration curve comparison between alanine dosimeters calibrated in the GC207 source

(5.052 Gy•s–1) and the GC232 source (1.001 Gy•s–1) in 2005.

Fig. 3. The relative difference of the computed absorbed dose from the absorbed dose delivered is plotted versus the absorbed dose delivered. The dose range is from 1 kGy to 50 kGy. The source of the absorbed-dose measurements used to compute the relative difference is detailed below. The open triangles are 2005 GC207-irradiated dosimeter absorbed doses computed from the 2004 calibration curve created with the GC232 source. The

open circles are 2005 GC207-irradiated dosimeter absorbed doses computed from the 2005 calibration curve created with the GC207 source, and represent the residuals of that curve. The filled triangles are 2005 GC232-irradiated dosimeter absorbed doses computed from the 2004 calibration curve created with the GC232 source. The

filled circles are 2005 GC232-irradiated dosimeter absorbed doses computed from the 2005 calibration curve

created with the GC207 source. The error bars represent the standard uncertainty (k = 1).
interpolation step and the saturation of the response in

this dose range.

No rate effect is detectable at 1 kGy, and the largest

effect is measured above 20 kGy. However, the data at

10 kGy for Fig. 3 and Fig. 4 were not consistent with

the measurement trend for doses above and below

10 kGy. A series of additional GC232/GC207 comparisons were measured to better characterize the transition between equivalence and non-equivalence. Figure

5 is a compilation of the Fig. 4 data and additional

measurements. From these data, the effect is seen to be

significant above 5 kGy.

The 2005 results show that the alanine dosimeters

irradiated in the lower-dose-rate (1.001 Gy•s–1) GC232

give a response equivalent to alanine dosimeters irradiated in the higher-dose-rate (5.052 Gy•s–1) GC207 if the

absorbed dose is under 5 kGy. For doses above 5 kGy,

the responses of the GC232 and GC207 dosimeters are

not equivalent, and the difference in the responses

increases with absorbed dose, reaching a maximum

above 20 kGy. Of particular interest is the equivalence

of dosimeters irradiated (>5 kGy) in the GC232 in 2004

and GC207 in 2005. The dose rate for the GC232 in

August 2004 was 1.093 Gy•s–1, while the dose rate for

the GC207 in April 2005 was 5.052 Gy•s–1. However,

after the GC232 rate decayed to 1.001 Gy•s–1 in 2005,

the response of GC232-irradiated dosimeters was ≈2 %

(for dosimeters >20 kGy) lower than that of dosimeters

irradiated in the GC207 at 5.052 Gy•s–1. The comparison of GC232 dosimeters from April 2005 (1.001

Gy•s–1) and August 2004 (1.093 Gy•s–1) follow this

trend of non-equivalence. However, the GC207-irradiated dosimeters in April 2005 (5.052 Gy•s–1) are equivalent to the GC232-irradiated dosimeters from August

2004 (1.093 Gy•s–1). This relation of the equivalence

between the response (R) of (>5 kGy) irradiated

dosimeters can be summarized as:

R(GC232/’05; 1.00 Gy•s–1) ≠ R(GC232/’04; 1.09

Gy•s–1) = R(GC207/’05; 5.05 Gy•s–1) = R(GC207/’04;

5.51 Gy•s–1)

Regulla and Deffner had claimed alanine dosimetry to

be independent of absorbed dose rate [14]. In that work,

alanine dosimeters irradiated to 5 kGy, 30 kGy,

100 kGy, and 350 kGy were irradiated with gamma

sources of dose rates between 0.028 Gy•s–1 and

28 Gy•s–1. Because of the low resolution (a small figure

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85

0.96

0.97

0.98

0.99

1.00

1.01

0 10 20 30 40 50 60

Absorbed Dose (kGy)

Dosimeter Response Ratios

207('05)/232('04)

"207('05)/207('05)"

232('05)/232('04)

232('05)/207('05)

Fig. 4. The alanine dosimeter measured response plotted as a ratio to the calibration curve response. The dose

range is from 1 kGy to 50 kGy. The source of the ratio measurements is detailed below. The open triangles are

2005 GC207-irradiated dosimeter measurements plotted as a ratio to the 2004 calibration curve created with the

GC232 source. The open circles are 2005 GC207-irradiated dosimeter measurements plotted as a ratio to the

2005 calibration curve created with the GC207 source, and are displayed for consistency with Fig. 2. The filled

triangles are 2005 GC232-irradiated dosimeter measurements plotted as a ratio to the 2004 calibration curve created with the GC232 source. The filled circles are 2005 GC232-irradiated dosimeter measurements plotted as a

ratio to the 2005 calibration curve created with the GC207 source. The error bars represent the standard uncertainty (k = 1).
with logarithmic scales) of the graphically displayed

data in that publication [14], it remains unclear if the

work presented here contradicts or is consistent with

the work of Regulla and Deffner. It is not possible to

reconcile these findings because the present work did

not cover the full range of dose rates previously studied, and the standard uncertainty (0.5 %; k = 1) of the

measurements presented here is far lower than the standard deviation (≈ 5 %) indicated in the plot of the previous data [14]. However, the measurement data

between 0.28 Gy•s–1 and 2.8 Gy•s–1 in the author’s plot

[14] are comparable to the present work and display a

trend consistent with the present work. The data of

Regulla and Deffner appear to show a lower alanine

response for dosimeters irradiated at 0.28 Gy•s–1 relative to those at 2.8 Gy•s–1. To be conclusive, the dose

rate range of the present study would need to be

expanded beyond current capabilities.

The data presented here suggest that, for absorbed

doses higher than 5 kGy, a rate dependence for alanine

is measurable. Moreover, Figs. 3, 4, and 5 demonstrate

that the sensitivity of alanine to dose rate (at these

doses) becomes significant somewhere between

1.1 Gy•s–1 and 1.0 Gy•s–1. Though confirmed by a significant number of measurements (Fig. 5), this assertion had to be tested rigorously. Was the effect:

• due to an undetectable mechanical error in the

GC232 and GC207?

• unique to the form/composition of the alanine

dosimeters used?

• unique to the NIST dosimetry system?

To address these questions, a series of tests using additional gamma sources, other forms of alanine dosimeters, and other dosimetry systems were undertaken.

3.2 NIST Gamma-Source Comparisons

The dose/dose-rate effect measurements triggered

recollections of previous inconsistencies in source

comparisons in 1995 and 2002. In the early days of

using alanine dosimetry for internal comparisons at

NIST, there was a reliance on expert opinions and publications for certain aspects of the system. One of these

aspects was dose rate. Archival publications by recognized experts in alanine dosimetry had found no

dependence of the alanine dosimeter on the absorbeddose rate [14]. Consequently, discrepancies that arose

in irradiations of alanine dosimeters at different rates

were attributed to other potential sources of error (e.g.,

environmental influences).

One such discrepancy was noted in 1995. As mentioned in Sec. 2.3, the dose rates for gamma sources

used in the NIST calibration scheme were characterized

by ratios of the alanine dosimeter responses at doses of

1 kGy and below. However, during an internal calibration in 1995, after multiple measurements at 1 kGy

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Fig. 5. GC232-irradiated alanine dosimeter measurements computed as a ratio to GC207-irradiated dosimeter

measurements for the same absorbed dose. The trend line is inserted as a visual aid and is not intended to represent any specific mathematical relation. The error bars represent the standard uncertainty (k = 1).
proved reproducible, a check of source comparisons at

25 kGy proved inconsistent with the 1 kGy data (see

Table 1). This discrepancy was attributed to unknown

environmental influences that presumably influenced

the 25 kGy measurements, but not the 1 kGy measurements. Since expert consensus was that dose rate was

not an influence on alanine dosimetry, the issue was not

pursued further at that time.

Table 1. Alanine dosimeter response ratios for source comparisons

in the 1995 internal calibration. Gamma source identifiers are accompanied by their dose rate in Gy•s–1. The values are accompanied by a

Type A uncertainty representing the measurement standard deviation. 

Absorbed Pool (0.3783) GC45 (1.045)

Dose (kGy) GC232 (3.453) GC232 (3.453)

1.0 1.007 ± 0.004 0.999 ± 0.004

25 0.990 ± 0.004 0.986 ± 0.004

The dose-dependent dose-rate effect was again

observed in 2002 during another internal calibration

(Table 2). Similar results were obtained. There was no

observable rate dependence in the alanine response at

1 kGy, but once again at 25 kGy the dosimeters irradiated in the low-rate source under-responded compared

to those irradiated in the highest-rate source (GC207).

Table 2. Alanine dosimeter response ratios for source comparisons

in the 2002 internal calibration. Gamma source identifiers are accompanied by their dose rate in Gy•s–1.

3 The values are accompanied by

a Type A uncertainty representing the measurement standard deviation.

Absorbed Pool (0.1544) GC45 (0.4242) GC232 (1.404)

Dose (kGy) GC207 (5.348) GC207 (5.348) GC207 (5.348)

1.0 0.996 ± 0.004 1.007 ± 0.004 1.004 ± 0.004

25 0.977 ± 0.004 0.986 ± 0.004 0.992 ± 0.004

Resolving these discrepancies was given a low priority because of the significant amount of reproducible

1 kGy comparison data obtained over the course of several years. The differences at 25 kGy were attributed to

minor environmental effects related to low-rate longduration irradiations to high doses. It was thought then

that because low-rate sources would not be selected for

high-dose irradiations that there would be no impact, or

at least an impact that was recognizable and avoidable.

What contributed to the recognition of this dose/doserate effect was the installation of a quality system at

NIST in 2004. Prior to 2004, calibrations of the alanine

dosimetry system were more frequent; thus the probability of the effect being observed was low. The frequent pre-2004 recalibration events were accompanied

by EPR maintenance routines (e.g., removal/cleaning

of EPR sample holders) that prevented long-term comparison measurements. In 2004 NIST began extending

the utility of the system calibration to longer times that

allowed NIST to directly compare measurements made

over months (in this case eight).

Following the 2005 recognition of the dose/dose-rate

effect, several additional comparisons of NIST low-rate

sources with the GC207 source were conducted during

2005 through 2007. One example of the data from this

period is shown in Table 3; these data correlate well

with the previous findings in Tables 1 and 2. The 2005-

through-2007 source-comparison measurements are

included in Fig. 6 with the results of the 1995 and 2002

data. Figure 6 plots the response ratios of dosimeters

irradiated with gamma sources of dose rates lower than

1.4 Gy•s–1 (these include the Pool, GC45, and GC232)

to dosimeters irradiated concurrently4 with gamma

sources of dose rates greater than 3.5 Gy•s–1. 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 measurements to

high-rate measurements remain clustered about unity

consistently from 1995 through 2007 regardless of the

gamma source used. The 20 kGy to 30 kGy measurement ratios consistently indicate a response 1 % to 2 %

lower than that from the 1 kGy measurements regardless of the gamma source used. These data clearly

demonstrate the reproducibility of the dose-dependent

dose-rate effect; for more than a decade from 1995 to

2007, dosimeters irradiated to doses higher than

20 kGy with gamma sources of 1.4 Gy•s–1 or less gave

a lower response than dosimeters irradiated with the

same gamma sources to 1 kGy.

Table 3. Alanine dosimeter response ratios for 2006 source comparisons. Gamma source identifiers are accompanied by their dose rate

in Gy•s–1. The values are accompanied by a Type A uncertainty representing the measurement standard deviation.

Absorbed Pool (0.0994) GC45 (0.2744) GC232 (0.907)

Dose (kGy) GC207 (4.578) GC207 (4.578) GC207 (4.578)

1.0 1.001 ± 0.004 1.003 ± 0.004 0.996 ± 0.004

50 0.985 ± 0.004 0.988 ± 0.004 0.983 ± 0.004

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87

3 The similarity of the GC207 dose rates for 2002 and 2004 is not an

error. In mid 2004, the source was reloaded with higher activity

Co-60 and recalibrated in late 2004.

4 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.
3.3 Other Alanine Dosimeters

The alanine dosimeter comparisons discussed in Sec.

3.2 were conducted with alanine pellets from a single

manufacturer (Gamma Service, Germany). Therefore it

was important to examine whether the dose/dose-rate

effect was unique to this manufacturer, to pellets in

general, or to any other alanine dosimeter type. Figure

7 summarizes the study of several alanine dosimeters

irradiated to high and low doses in the GC232 and

GC207. The dose rates for the GC207 and GC232 were

4.58 Gy•s–1 and 0.907 Gy•s–1, respectively. The alanine

dosimeters were from different manufacturers: Gamma

Service (Germany); Harwell (UK); NIST (USA);

Bruker (USA); and, NIM (National Institute of

Metrology, China).5 The study included alanine film

and several pellet types including two different lots of

Gamma Service pellets and NIM pellets made with

either of two alanine stereoisomer forms, L-alanine or

DL-alanine (D, L mixture). The alanine dosimeter

response ratio measured from dosimeters irradiated in

the low-rate GC232 source compared to the high-rate

GC207 source were consistently near unity for lowdose irradiations and significantly less than unity for

high-dose irradiations. This study demonstrates that the

dose/dose-rate effect is inherent to alanine itself regardless of the form or formulation of the dosimeter.

3.4 International Comparisons and Tests

The last comparison of the high-dose standards for

absorbed dose to water from 60Co gamma radiation

among the primary dosimetry laboratories maintaining

standards and services was performed in the late 1990s.

Organized by the Bureau International des Poids et

Mesures (BIPM, France) and conducted by the

National Institute of Standards and Technology (NIST,

USA) and the National Physical Laboratory (NPL,

UK), the comparison also included the Istituto

Nazionale di Metrologia delle Radiazioni Ionizzanti

(ENEA-INMRI, Italy), the Physikalisch-Technische

Bundesanstalt (PTB, Germany), the National Institute

of Metrology (NIM, China), and the International

Atomic Energy Agency (Vienna). Because it does not

offer a high-dose service, the BIPM took part at a single dose level (1 kGy) to provide a direct link to the

international reference for absorbed dose to water in

60Co beams. The comparison was recently published

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88

Fig. 6. Alanine dosimeter measurement ratios comparing low-dose-rate irradiations to high-dose-rate irradiations at the same dose (1 kGy: circles or 25 kGy: triangles). The measurements made in 1995, 2002, and 2005

are defined in the legend. Inserted into the plot are the dose rates of the high-rate sources in these respective

years.

5 NIST pellets were manufactured according to the published preparation [16]. NIM pellets were manufactured according to the published preparation [7]. The L and DL designation refers to the

stereoisomeric forms of alanine pellets used, L-alanine and DL-alanine, respectively. The quantity of available NIM pellets was insufficient to permit low-dose comparisons.
[13] and found that there was a general level of agreement among the institutes at three dose levels: 5 kGy,

15 kGy, and 30 kGy. The agreement was well within

the expanded uncertainty for each institute. However,

an examination of the level of agreement reveals suggestions of dose-dependent trends in the 5 kGy through

30 kGy data. Also noteworthy is the observation that

the best agreement between NIST and NPL occurs at

1 kGy. It is possible that this unknown effect impacted

the calibration of high-dose sources maintained by

national laboratories. Unfortunately, further analysis of

this aspect of the data is not possible because the dose

rates for 60Co sources within a laboratory’s calibration

scheme and the protocol for calibration of these sources

are unpublished. It is hoped that future comparisons

will include these details so that this effect can be studied in detail.

Periodic comparisons with other NMI’s is an important component of the NIST IRD Quality System.

Historically, NIST has performed high-dose comparisons with NPL. The goal is to perform these comparisons at least biannually. For these comparisons, NIST

and NPL exchange 12 alanine dosimeter vials, irradiate

four vials to each of three doses (5 kGy, 15 kGy,

30 kGy), and return the vials to the issuing laboratory

for analysis. Typically, there is a 1 % to 2 % relative

difference between the absorbed dose as measured by

the analyzing laboratory and the absorbed dose delivered by the irradiating laboratory. The results from the

most recent comparison in December 2004 are displayed in Fig. 8. Because prior to this comparison NIST

had recently changed its calibration scheme to employ

the Pool source for routine internal comparisons, NIST

made a small modification to the protocol. Instead of

irradiating four dosimeter vials to 5 kGy, 15 kGy, and

30 kGy in its highest rate source (GC207, 5.052 Gy•s–1),

NIST irradiated three vials to each of the three prescribed doses and irradiated the remaining three vials to

7 kGy in the lower rate Pool source (0.110 Gy•s–1).

Figure 8 shows that the 5 kGy, 15 kGy, and

30 kGy GC207-irradiated dosimeters, as measured by

NPL, are relatively consistent with regard to the relative difference (approximately 1.5 %) between the

NPL-assessed dose and the NIST-delivered dose.

However, the Pool-irradiated dosimeters deviated from

this trend and yielded a relative difference of approximately 2.3 %. The net 0.8 % relative difference is consistent with the rate effect measured at NIST for 7 kGy

(Fig. 5).

The final step in the dose/dose-rate study is independent verification of the measurements made at

NIST. To verify the effect, NPL kindly agreed to supply

and measure alanine and dichromate dosimeters to be

irradiated by NIST. Using the NIST five-position holdVolume 113, Number 2, March-April 2008

Journal of Research of the National Institute of Standards and Technology

89

0.950

0.960

0.970

0.980

0.990

1.000

GS 20604

Pellets

GS 30901

Pellets

Harwell Pellets NIST Pellets NIM L-Alanine

Pellets

NIM DL-Alanine

Pellets

Alanine Film

Alanine Dosimeter Type/Formulation

Ratio GC232/GC207 Alanine Response

1-2 kGy

50 kGy

Fig. 7. Alanine dosimeter response ratios for GC232/GC207 source comparisons for a variety of alanine

dosimeter types and formulations. The dose rates for the GC207 and GC232 sources were 4.58 Gy•s–1 and

0.907 Gy•s–1, respectively.
er, the experimental design for each run was to irradiate

two NPL dichromate dosimeter ampoules and two NPL

alanine dosimeter vials, with a NIST alanine dosimeter

vial in the fifth position.6 Two sources were used for the

irradiation, the GC207 (4.578 Gy•s–1) and the GC232

(0.907 Gy•s–1). The doses selected for each source were

10 kGy and 50 kGy (10 kGy was the lowest dose possible dose for this dichromate dosimeter formulation).

The ratio of the mean NPL-measured dose (9.79 kGy;

uncertainty of 1.2 %, k = 1) for the two alanine vials

irradiated to 10 kGy in the GC232 source to the mean

NPL-measured dose (9.91 kGy) for the two alanine

vials irradiated to 10 kGy in the GC207 source was

0.988 (Fig. 9). The ratio of the mean NPL-measured

dose (9.79 kGy; uncertainty of 1.04 %, k = 1) for the

two dichromate ampoules irradiated (together with the

NPL alanine cited above) in the GC232 source to the

mean NPL-measured dose (9.89 kGy) from the two

dichromate ampoules irradiated (together with the NPL

alanine cited above) in the GC207 source was 0.990 ±

0.008. The alanine dosimeter GC232/GC207 response

ratio for NPL dosimeters measured above was 0.987 ±

0.009. The alanine dosimeter response GC232/GC207

ratio for NIST dosimeters irradiated simultaneously

with the NPL dosimeters to 10 kGy was 0.996 ± 0.004;

the GC232/GC207 ratio for NIST-measured absorbed

dose was 0.995 ± 0.007.

The ratio of the mean NPL-measured dose

(48.7 kGy) from the two alanine vials irradiated to

50 kGy in the GC232 source to the mean NPL-measured dose (50.3 kGy) from the two alanine vials irradiated to 50 kGy in the GC207 source was 0.967 (Fig. 9).

The ratio of the mean NPL-measured dose (49.3 kGy)

from the two dichromate ampoules irradiated (together

with the NPL alanine cited above) in the GC232 source

to the mean NPL-measured dose (49.4 kGy) from the

two dichromate ampoules irradiated (together with the

NPL alanine cited above) in the GC207 source was

0.997 ± 0.008. The alanine dosimeter GC232/GC207

response ratio for NPL dosimeters measured above was

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90

Fig. 8. The relative difference of the NPL-computed absorbed dose from the absorbed dose delivered by NIST

is plotted versus the absorbed dose delivered. The open squares are 5 kGy, 15 kGy, and 30 kGy doses delivered

by the GC207 source. The filled squares represent a 7 kGy dose delivered by the Pool source. The open diamonds are NPL dichromate doses irradiated in the GC207 source to 10 kGy or 50 kGy. The filled diamonds are

NPL dichromate doses irradiated in the GC232 source to 10 kGy or 50 kGy. The open triangles are NPL alanine

doses irradiated in the GC207 source to 10 kGy or 50 kGy. The filled triangles are NPL alanine doses irradiated in the GC232 source to 10 kGy or 50 kGy. The × symbols are NIST alanine doses irradiated in the GC207

source to 10 kGy or 50 kGy. The * symbols are NIST alanine doses irradiated in the GC232 source to 10 kGy

or 50 kGy. The standard uncertainties (k = 1) for these measurements are: NPL alanine, 1.2 %; NPL dichromate,

1.04 %; NIST alanine, 1.0 %.

6 An examination of the position dependence with alanine dosimeters

for this five-position geometry in the GC232 and GC207 revealed a

relative standard deviation of 0.50 % and 0.25 % respectively.
0.977 ± 0.009. The alanine dosimeter response

GC232/GC207 ratio for NIST dosimeters irradiated

simultaneously with the NPL dosimeters to 50 kGy was

0.985 ± 0.004; the GC232/GC207 ratio for NIST-measured absorbed dose was 0.976 ± 0.007.

Though slightly higher than the previously determined ratios shown in Fig. 5, the NIST response ratios

at 10 kGy (0.996) and 50 kGy (0.985) are consistent

with the previous findings in that the 50 kGy ratio is

about 1 % lower than the 10 kGy ratio. The NIST alanine, NPL alanine and NPL dichromate dose ratios are

consistent with each other at 10 kGy. However, at

50 kGy the NPL dichromate system measured the

absorbed dose for dosimeters irradiated in the GC232

and GC207 sources to be equivalent, while NPL alanine dosimeters irradiated simultaneously with the

NPL dichromate dosimeters measured an approximately 2 % lower dose in the GC232 source as compared to

the GC207 source NPL alanine dosimeter dose measurement. So, whereas the 50 kGy NPL dichromate

dosimeter showed no effect of irradiation at different

rates, the NPL alanine dosimeter measurements confirmed the rate effect documented by NIST alanine

dosimeter measurements and are consistent with all

other alanine dosimeter types shown in Fig. 7.

3.5 EPR Studies

Elucidation of the physico-chemical mechanism for

the dose/dose-rate dependence is beyond the scope of

this study; however, a series of EPR investigations

were undertaken to learn if spectroscopic data could

support the hypothesis that the differences in EPR signal amplitude are related to differences in the free-radical composition of alanine dosimeters irradiated to

identical doses but at different dose rates.

Certain aspects of alanine dosimetry suggest different behavior of the chemistry at low doses versus high

doses. For example, there is a marked difference in the

temporal changes of the EPR signal amplitude after

irradiation below and above 1 kGy [8]. Though no significant change in the EPR spectrum was measurable at

high doses compared to low doses, low-intensity signal

contributions to the overall spectrum from secondary or

tertiary radicals could not be ruled out. Early experimental studies using photo-bleaching [16] and spintrapping [17] of irradiated alanine were able to detect

spectroscopically different radicals at high doses. Later,

advances in EPR spectral analysis identified a secondary radical and postulated the existence of a third [18].

A dose dependence of the relative proportions of these

radicals has not been reported.

Starting with the assumption that one or more of the

alanine free radicals is rate sensitive and that rate senVolume 113, Number 2, March-April 2008

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91

Fig. 9. Dosimeter ratios for GC232/GC207 source comparisons for: NPL dichromate doses, NPL alanine doses, NPL alanine EPR amplitudes, NIST alanine EPR amplitudes, and NIST alanine doses. The

black bars are 10 kGy ratios and the white bars are 50 kGy ratios. The standard uncertainties (k = 1) for

these measurements are: NPL alanine, 1.2 %; NPL dichromate, 1.04 %; NIST alanine, 1.0 %.
sitivity is dose dependent, experiments were conducted

that sought measurable spectroscopic differences in the

EPR spectrum. There are several EPR parameters that

vary with the molecular composition or configuration

of a free radical. The most common of these, g-factor

and hyperfine coupling constants, are not well resolved

because the irradiated alanine spectrum is composed of

two or more radicals with multiple broad resonances.

Microwave power saturation behavior is also known to

be dependent on the form of the free radical. The EPR

signal amplitude is proportional to the square root of

the applied microwave power, being approximately linear with power until saturation is achieved at higher

power. For different free radicals, the power dependence can exhibit differences in the power value at maximum EPR amplitude and in the general shape of the

EPR amplitude versus power curve over the range of

applied microwave power. Based on the magnitude of

the effect measured in this study, these differences were

assumed to be relatively small, so rather than compare

absolute differences a comparison of the ratios of the

power dependences for high- and low-dose-rate irradiated alanine pellets was measured.

The absorbed doses selected for the study were:

100 Gy, 1 kGy, 5 kGy, 20 kGy, and 50 kGy. At each

dose level, a four-dosimeter vial was irradiated simultaneously in both the GC45 source (0.873 Gy•s–1) and

the GC207 source (4.41 Gy•s–1). The irradiation timers

were set such that the irradiations ended at approximately the same time. The dosimeters were measured

between 24 hours and 48 hours after irradiation. For

each of the four dosimeters irradiated to the same dose

but with different dose rates, the signal amplitude was

computed as a function of microwave power. The

microwave power was varied by stepping the attenuator in 1 dB increments to yield 21 power levels from

0.5 mW to 50 mW.7

The microwave power dependence for each dosimeter was recorded and combined as a ratio for dosimeters

irradiated with either of the two gamma sources. Figure

10 shows a plot of representative measurements from

these combined ratios, but only for dosimeters irradiatVolume 113, Number 2, March-April 2008

Journal of Research of the National Institute of Standards and Technology

92

7 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 [10]). Other alanine EPR recording parameters include:

frequency, 9.684 GHz; center field, 345.5 mT; magnetic-field sweep

width, 1.0 mT; modulation amplitude, 0.2 mT; time constant, 1.3 s;

sweep time, 41.9 s.

Fig. 10. Alanine EPR amplitude measurement ratios for individual dosimeters irradiated in like sources (e.g.,

GC207 compared to GC207) for doses from 100 Gy to 50 kGy. The EPR amplitudes are plotted against the

square root of the incident microwave power of the EPR spectrometer. The GC45/GC45 ratios are the × symbol

and open symbols. The GC207/GC207 ratios are the + symbol and filled symbols.
ed with identical rates. GC207 dosimeters were compared to other GC207 dosimeters and the same for

GC45 dosimeters. There are no observable trends

regardless of the gamma source at each dose level or

between dose levels. However, when ratios are computed for dosimeters irradiated to the same dose with different dose rates (Fig. 11) there is a significant divergence in the data that increases with increasing

absorbed dose. For the response ratios measured in Fig.

5, the dose/dose-rate effect on alanine becomes significant above 5 kGy. This trend holds true in Fig. 11, with

significant differences in the power dependence at

20 kGy and 50 kGy. The 100 Gy and 1 kGy

GC207/GC45 ratios in Fig. 11 are consistent with the

100 Gy and 1 kGy data in Fig. 10, where the ratios were

for dosimeters of the same dose and dose rate. The

GC207/GC45 ratio at 5 kGy, as in Fig. 5, suggests the

onset of change. At 20 kGy and 50 kGy the ratios rise

above unity because of the reduced EPR amplitude for

the GC45 irradiated dosimeters at high dose. In addition, the GC207/GC45 ratio of EPR amplitudes for

dosimeters irradiated to 20 kGy and 50 kGy increases

with microwave power. This is a result of the differences in the microwave-power saturation curves for the

different radical distributions contained in the respective dosimeters. This effect indicates that the free-radical composition of the high- and low-dose-rate irradiated alanine dosimeters is different at doses higher than

5 kGy. This does not necessarily imply different (or

new, unique) radicals, but could indicate different relative concentrations of the radicals that are present at

lower doses. The EPR signal amplitude of the central

resonance in the alanine spectrum is a combination of

the EPR intensities from two or more free radical types.

The relative sensitivity of each free-radical type to

microwave power produces changes in the overall EPR

intensity that results from the combination of the individual intensities of each radical type.

It may be possible to exploit the relative response

differences that are dependent on microwave power to

discern whether measurements of dosimeters irradiated

above 5 kGy are influenced by the rate effect. During

industrial processing the movement of product accompanied by dosimeters to, from, and about the radiation

source exposes the dosimeters to a broad range of dose

rates. The extent to which, as part of this process, low

dose-rate exposures contribute to the total absorbed

dose and influence the dose assessment is unknown. A

microwave-power dependence comparison of dosimeters irradiated in an industrial facility to calibrated

dosimeters irradiated at a fixed (high) dose rate may be

a diagnostic tool to explore rate-effect influences in

industrial radiation dosimetry.

Volume 113, Number 2, March-April 2008

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93

Fig. 11. Alanine EPR amplitude measurement ratios for individual dosimeters irradiated in different sources

(GC207 compared to GC45) for doses from 100 Gy to 50 kGy. The EPR amplitudes are plotted against the

square root of the incident microwave power of the EPR spectrometer. The absorbed doses for the GC207/GC45

ratios are given in the legend.
4. Summary

This work characterizes a complex relation between

the radiation chemistry of crystalline alanine and the

applied dose rate as a function of absorbed dose. This

effect appears intrinsic to alanine itself and is not

dependent on the form or formulation of the alanine

dosimeter. These studies suggest that the production of

one or more of the alanine radicals is dependent on the

dose rate and that this rate effect becomes significant

above 5 kGy. These data are certainly consistent with

other alanine dose dependent effects that have been

observed in the kilogray range. Dose-dependent behavior has been documented for effects based on temperature [6], time [8], and relative humidity [9]. The boundary for all of these effects is above 1 kGy and may point

to a common origin in the chemistry of the different

free radicals that combine to form the measured spectrum.

Typically high-dose-rate calibration sources maintained by 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

for a high-rate gamma calibration source by comparison 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. For the NMI that

has ensured that their dose rates are not influenced by

this dose-rate effect, two considerations remain. First,

caution would be advised to avoid using alanine

dosimeters at high doses (> 5 kGy) for transfer calibrations of low-dose-rate (< 2 Gy•s–1) sources and second

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

calibrate alanine dosimeters.

It is a distinct possibility that high-dose calibration

services provided by other NMIs and/or secondary

standards laboratories are in error due to the rate effect,

and that this error has been propagated to customers.

These laboratories should perform internal system

checks that use the experimental design employed in

the present work, and follow that with comparisons to

verify their rates. Considering that the previous international high-dose dosimetry comparison [13] did not

take this effect into account, it is recommended that for

follow-up NMI comparisons the calibration schemes of

the participants should be examined prior to starting.

Acknowledgments

The authors thank Drs. Peter Sharpe and David

Crossley of the National Physical Laboratory (UK) for

supplying alanine and dichromate dosimeter measurement services and Drs. Min Lin, Ying Cui, and

Kesheng Chen of the China Institute of Atomic Energy

for supplying L- and DL-alanine dosimeters.

5. References

[1] American Society for Testing and Materials, Standard Practice

for Use of the Alanine-EPR Dosimetry System, International

Standards Organization & American Society for Testing and

Materials, Designation ISO/ASTM 51607: 2004(E).

[2] S. R. Domen, A sealed water calorimeter for measuring

absorbed dose, J. Res. Natl. Inst. Stand. Technol. 99, 121-141

(1994).

[3] J. C. Humphreys, J. M. Puhl, S. M. Seltzer, W. L. McLaughlin,

M. F. Desrosiers, D. L. Bensen, and M. L. Walker, Radiation

Processing Dosimetry Calibration Services and Measurement

Assurance Program, 1998 NIST Special Publication 250-44.

[4] W. L. McLaughlin and M. F. Desrosiers, Dosimetry systems for

radiation processing, Radiat. Phys. Chem. 46, 1163-1174

(1995).

[5] P. Sharpe and J. Sephton, Therapy level alanine dosimetry at the

NPL, Proceedings of Alanine Dosimetry for Clinical

Applications, Physikalisch-Technische Bundesanstalt Report,

PTB-Dos-51 (2006) 3-8.

[6] M. F. Desrosiers, S. L. Cooper, J. M. Puhl, A. L. McBain, and

G. W. Calvert, A study of the alanine dosimeter irradiation temperature coefficient in the –77 °C to +50 °C range, Radiat.

Phys. Chem. 71, 365-370 (2004).

[7] M. F. Desrosiers, M. Lin, S. L. Cooper, Y. Cui, and K. Chen, A

study of the irradiation temperature coefficient for L-alanine

and DL-alanine dosemeters, Radiat. Prot. Dosim. 120, 235-237

(2006).

[8] V. Yu. Nagy and M. F. Desrosiers, A complex time dependence

of the EPR signal of irradiated L-α-alanine, Appl. Radiat. Isot.

47, 789-793 (1996).

[9] O. F. Sleptchonok, V. Y. Nagy, and M. F. Desrosiers,

Advancements in the accuracy of the alanine dosimetry system,

Part 1. The effects of environmental humidity, Radiat. Phys.

Chem. 57, 115-133 (2000).

[10] V. Yu. Nagy, O. F. Sleptchonok, M. F. Desrosiers, R. T. Weber,

and A. H. Heiss, Advancements in accuracy of the alanine

dosimetry system, Part 3. Usefulness of an adjacent reference

sample, Radiat. Phys. Chem. 59, 429-441 (2000).

[11] M. F. Desrosiers, Dose Interpretation of Customer-Irradiated

NIST Transfer Dosimeters, Ionizing Radiation Division Quality

Manual, NIST Physics Laboratory, http://www.physics.nist.

gov/Divisions/Div846/QualMan/procedures.html (Version

3.01) [Accessed February 1, 2008].

[12] J. C. Humphreys, J. M. Puhl, S. M. Seltzer, W. L. McLaughlin,

M. F. Desrosiers, V. Y. Nagy, D. L. Bensen, and M. L. Walker,

Radiation Processing Dosimetry Calibration Services: Manual

of Calibration Procedures, NIST Special Publication 250-45,

March 1998.

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[13] D. T. Burns, P. J. Allisy-Roberts, M. F. Desrosiers, V. Yu. Nagy,

P. H. G. Sharpe, R. F. Laitano, K. Mehta, M. K. H. Schneider,

and Y. Zhang, CCRI supplementary comparison of standards

for absorbed dose to water in 60Co gamma radiation at radiation

processing dose levels, Radiat. Phys. Chem. 75, 1087-1092

(2006).

[14] D. F. Regulla and U. Deffner, Dosimetry by ESR spectroscopy

of alanine, Appl. Radiat. Isot. 33, 1101-1114 (1982).

[15] D. Nichiporov, V. Kostjuchenko, J. Puhl, D. Bensen, M.

Desrosiers, C. Dick, W. McLaughlin, T. Kojima, B. Coursey,

and S. Zink, Investigation of applicability of alanine and

radiochromic detectors to the dosimetry of proton clinical

beams, Appl. Radiat. Isot. 46, 1355-1362 (1995).

[16] A. Wieser, C. Lettau, U. Fill, and D. F. Regulla, The influence

of non-radiation induced ESR background signal from paraffinalanine probes for dosimetry in the radiotherapy dose range,

Appl. Radiat. Isot. 44, 59-65 (1993).

[17] M. F. Desrosiers, G. Burlinska, P. Kuppusamy, J. Zweier, D. M.

Yaczko, F. P. Auteri, M. R. McClelland, C. E. Dick, and W. L.

McLaughlin, Research and development activities in electron

paramagnetic resonance dosimetry, Radiat. Phys. Chem. 46,

1181-1184 (1995).

[18] E. Malinen, M. Z. Heydari, E. Sagstuen, and E. O. Hole,

Alanine radicals, Part 3. Properties of the components contributing to the EPR spectrum of x-irradiated alanine dosimeters, Radiat. Res. 159, 23-32 (2003).

About the authors: Marc Desrosiers (chemist), James

Puhl (physical scientist), and Sarenée Cooper (physicist) are in the Ionizing Radiation Division of the NIST

Physics Laboratory. The National Institute of

Standards and Technology is an agency of the U.S.

Department of Commerce.

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