Advancements in accuracy of the alanine dosimetry system.

Part 1. The eects of environmental humidity

Olga F. Sleptchonok*, Vitaly Nagy, Marc F. Desrosiers

Ionizing Radiation Division, National Institute of Standards and Technology, Gaithersburg, MD, 20899-8460, USA

Received 5 April 1999; received in revised form 26 April 1999; accepted 27 April 1999

Abstract

A one-year study of the EPR signal of g-irradiated (60Co) L-a-alanine with simultaneous monitoring of the cavity

Q-factor was undertaken. The widespread opinion that the EPR signal remains absolutely stable under normal

laboratory storage conditions is inaccurate. At 0% humidity, the signal can be regarded as stable within 21% of its

initial value for 6 months for 1 and 10 kGy doses, but for only 3 months for 100 kGy. When stored at the same

relative humidity values up to 60%, the fading rates for dosimeters irradiated to 1 and 10 kGy are similar, whereas

signals of dosimeters irradiated to 100 kGy fade considerably faster for all humidities. The rates of fading increase

with the relative humidity, especially above 60% R. H. Environmental humidity also deteriorates the accuracy of

alanine dosimetry by changing the resonant cavity Q-factor. This is particularly important when irradiated alanine

dosimeters are used as instrument calibration standards. Short-term changes in alanine EPR signal amplitudes were

recorded upon removal of the irradiated dosimeters from their storage environments. The importance of an in situ

standard to correct for measurement errors due to environmental eects is demonstrated. # 2000 Elsevier Science

Ltd. All rights reserved.

Keywords: Alanine; EPR dosimetry; Humidity; Fading

1. Introduction

Changes in the radiation-induced electron paramagnetic resonance (EPR) signal between the irradiation

and subsequent EPR measurements are an important

factor directly aecting the accuracy of alanine-EPR

dosimetry. It is well known that radiation-induced radicals in alanine are far more stable than the free radical

species produced by ionizing radiation in most other

organic substances; this is the primary reason for alanine being the most commonly used material for highdose EPR dosimetry. However, the stability of the alanine radicals is not absolute, and they are known to

decay at elevated temperatures and higher humidity.

Over the past two decades, many sources of inaccuracies in alanine dosimetry have been successfully eliminated such that errors even below even 1% are

signi®cant. Therefore, EPR dosimetrists now need very

precise information on the stability of alanine radicals

under normal environmental conditions of dosimetry.

Previous published studies addressing EPR signal

stability of irradiated alanine are summarized in

Tables 1 and 2. The prevailing opinion is that the

degree of fading for the alanine signal, under normal

laboratory conditions, remains below 1% for at least 1

year. Unfortunately, the studies claiming high stability

generally provide neither enough original numeric

Radiation Physics and Chemistry 57 (2000) 115±133

0969-806X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 9 - 8 0 6X ( 9 9 ) 0 0 3 3 8 - 2

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* Corresponding author. Tel.: +1-301-975-2539; Fax: +1-

301-869-7682.

E-mail address: olga.sleptchonok@nist.gov (Olga Sleptchonok)
Table 1

Reported results of the studies of alanine signal fading at room temperature: Experiments with controlled humidity conditionsAuthor/Year Dosimeter

characteristics

Dose(kGy)a

Relativehumidity(%)

Period ofobservation(month)

Fading(%)

Numberof measuredsessions

Uncertainty(%)b

Referencesampleused

Comments

Dener and Regulla(1980)

?-Ala+paran(?+?);D = 5 mm,h = 7.5 mm

1±100 40±60 1

00c NAd NA NA

1±5 70 1 0 NA NA NA20 70 1 2 NA NA NA100 70 1 5 NA NA NA1±5 90±100 1 3 NA NA NA20 90±100 1 4 NA NA NA100 90±100 1 7 NA NA NA

Regulla and Dener(1982)

L-Ala+paran

(90+10);D = 4.9 mm,h = 10 mm

1 40±60 1

00 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

5 40±60 1

00 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

25 40±60 1

00 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

40 40±60 1

00 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

100 40±60 1

00 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

1 70 1

00 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

5 70 1 0.5 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

25 70 1 2 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

116 O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133
Table 1 (continued )

Author/Year Dosimeter

characteristics

Dose(kGy)a

Relativehumidity(%)

Period ofobservation(month)

Fading(%)

Numberof measuredsessions

Uncertainty(%)b

Referencesampleused

Comments

40 70 1 3 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

100 70 1 6 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

1 90 1 2 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

5 90 1 3 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

25 90 1 5 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

40 90 1 6 5 3±5 Alanine pellet, checks before

and after each series ofmeasurements

100 90 1 8 5 3±5NA Below room

humidity

24 < 1 NA NA Alanine pellet, checks before

and after each series ofmeasurements

Stored at 68C

Regulla and Dener(1985)

L-a-Ala+paran

(90+10);D = 4.9 mm,h = 10 mm

NA 40±60 36 < 1 NA NA Alanine pellet, once a day

Kojima and Tanaka(1989)

DL-Ala+polystyrene

(70+30);D = 3 mm,h = 30 mm

NA 60 NA ``very stable'' NA NA NA

Kojima et al.(1992)

DL-aAla+polystyrene

(70+30);D = 3 mm,h = 30 mm

1.4 50 5.3

00 NA NA Alanine pellet, before and

after measurement series

14 50 3.3 < 2 NA NA Alanine pellet, before and

after measurement series

(continued on next page)

O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133 117
Table 1 (continued )

Author/Year Dosimeter

characteristics

Dose(kGy)a

Relativehumidity(%)

Period ofobservation(month)

Fading(%)

Numberof measuredsessions

Uncertainty(%)b

Referencesampleused

Comments

100 50 0.2 1 NA NA Alanine pellet, before and

after measurement series

100 50 5.3 6 NA NA Alanine pellet, before and

after measurement series

Arber and Sharpe(1993)

L-Ala+paran

(90+10);D = 5 mm,h = 2.5 mm

1 0 3.3 0 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 1 week

1 40 3.3 2.3 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 1 week

1 60 3.3 1.7 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 1 week

1 80 3.3 3.6 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 1 week

10 0 3.3 (0.4) 10 0.3±0.5 NA Preconditioned at

the same humiditybefore irradiationfor 1 week

10 40 3.3 1.7 10 1.0±1.3 NA Preconditioned at

the same humiditybefore irradiationfor 1 week

10 60 3.3 1.7 10 0.3±0.6 NA Preconditioned at

the same humiditybefore irradiationfor 1 week

10 80 3.3 2.2 10 1.0±1.3 NA Preconditioned at

the same humiditybefore irradiationfor 1 week

118 O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133
Table 1 (continued )

Author/Year Dosimeter

characteristics

Dose(kGy)a

Relativehumidity(%)

Period ofobservation(month)

Fading(%)

Numberof measuredsessions

Uncertainty(%)b

Referencesampleused

Comments

40e 0 3.3 (0.1) NA NA NA Preconditioned at

the same humiditybefore irradiationfor 1 week

40 40 3.3 1.5 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 1 week

40 60 3.3 1.6 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 1 week

40 80 3.3 2.9 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 1 week

1 0 3.3 (0.1)f NA NA NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

1 40 3.3 0.8 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

1 60 3.3 0.7 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

1 80 3.3 2.3 NA NA NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

10 0 3.3 0.3 10 0.5±1 NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

(continued on next page)

O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133 119
Table 1 (continued )

Author/Year Dosimeter

characteristics

Dose(kGy)a

Relativehumidity(%)

Period ofobservation(month)

Fading(%)

Numberof measuredsessions

Uncertainty(%)b

Referencesampleused

Comments

10 40 3.3 0.9 10 0.5±1 NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

10 60 3.3 0.6 10 0.5±0.7 NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

10 80 3.3 1.4 10 0.5±1 NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

40 40 3.3 1.1 10 0.5±1 NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

40 60 3.3 1.6 10 0.5±1 NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

40 80 3.3 1.9 10 0.5±1 NA Preconditioned at

the same humiditybefore irradiationfor 4 weeks

1 0 3.3 0.1 10 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

1 40 3.3 (0.4) 10 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

1 60 3.3 (0.1) 10 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

120 O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133
Table 1 (continued )

Author/Year Dosimeter

characteristics

Dose(kGy)a

Relativehumidity(%)

Period ofobservation(month)

Fading(%)

Numberof measuredsessions

Uncertainty(%)b

Referencesampleused

Comments

1 80 3.3 (1.1) 10 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

10 0 3.3 (0.4) 10 0.8±1 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

10 40 3.3 0.2 10 0.2±0.3 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

10 60 3.3 0.6 10 0.2±0.5 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

10 80 3.3 1.5 10 0.5±1.0 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

40 0 3.3 (0.5) 10 0.5±1.0 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

40 40 3.3 0.2 10 0.5±1.0 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

40 60 3.3 (0.4) 10 0.5±1.0 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

40 80 3.3 1.9 10 0.5±1.0 NA Preconditioned at

the same humiditybefore irradiationfor 8 weeks

(continued on next page)

O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133 121
Table 1 (continued )

Author/Year Dosimeter

characteristics

Dose(kGy)a

Relativehumidity(%)

Period ofobservation(month)

Fading(%)

Numberof measuredsessions

Uncertainty(%)b

Referencesampleused

Comments

Wieser et al. (1993) L-a-Ala+paran

(85+15);D = 4.9 mm,h = 10 mm

0.01 50 1.6

00 7 1 NA

0.01 80 1.6 13 7 1 NA 308C

Juncheng et al. (1996) L-a-Ala+paran

(?+?), rods;D = 4.5 mm,L = 10 mm

0.1 12 6.6 0 NA NA Alanine rod

33 6.6 0 NA NA Alanine rod

58 6.6 3 NA NA Alanine rod76 2.5 84 NA NA Alanine rod98 0.3 >98 NA NA Alanine rod

Kojima et al. (1997)(S]2000)

DL-aAla+polystyrene

(50+50);D = 3 mm,h = 20 mm

0.1±100 40 2 < 1 NA NA NA

DL-aAla+polystyrene

(50+50);D = 3 mm,h = 20 mm

1 40 12 < 1 NA NA NA

a 60g-Co gamma rays are assumed unless speci®ed otherwise in the ``Comments'' column. b In many cases, values in this column were determined from plots in small ®gures and, thus, should be regarded as very rough estimates.

c Signal changes are within uncertainties of the experiment.

d NA=no explicit information on this point is available in the publication.

e Values of fading for 40 kGy in this study are approximate because of the nonlinearity of the calibration plot in this range used for their determination. The error due to nonlinearity is likely to be very small as compared to other uncertainties. f Values in parentheses represent increase in the signal with respect to the initial value.

122 O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133
data, nor sucient experimental details to support the

validity of this statement. Indeed, it is dicult to

monitor reliably small changes in alanine signal amplitudes over extended periods of time. The necessary

long-term standardization or control of the EPR spectrometer sensitivity with a high accuracy is a formidable problem in itself. Yet, as can be seen from the

tables, most of the authors do not mention any reference samples. Many who monitor the sensitivity with a

reference sample use an irradiated alanine dosimeter

for this purpose; in principle the signal of such a

sample is prone to the same changes as signals to be

monitored. Even those rare EPR spectrometrists who

employed a non-alanine reference sample almost

invariably did not record the reference signal with the

alanine dosimeter simultaneously present in the cavity,

and, thus, did not take into account variations in the

cavity Q-factor. This parameter is particularly important because of the varying moisture content of the

dosimeters. The diculties in evaluating the reliability

of previous studies claiming extremely high signal stability are compounded by the relatively small number of

measurements taken and the considerable scatter of

data points in the published plots.

Our routine experience has suggested that the fading

rate of alanine radicals under ``normal laboratory conditions'' is actually somewhat higher than has been

commonly reported. This prompted us to undertake a

new investigation of the problem with special attention

to several important experimental details and the acquisition of a large number of measurements sucient

for statistically signi®cant conclusions.

2. Experimental1

2.1. Controlled humidity environments

The humidity in an enclosed volume above a saturated aqueous solution of a particular salt in a closed

space depends only on temperature (Rockland, 1960);

this principle is used to create environments with standardized humidities. An important advantage of such

three-phase systems over the two-phase ones, such as

sulfuric acid or glycerol, is the independence of the

relative humidity produced from moisture being

absorbed or evolved by substances stored in the

chamber. Of the numerous salts recommended for this

purpose, we selected those exhibiting the weakest temperature dependence in the typical room temperature

range (Table 3). Because the temperature in our laboratory remained within the 22±258C range, these systems reliably provided constant relative humidities (R.

H.) accurate to 21%. Polycarbonate desiccators

(Nalgene) were used as controlled humidity chambers.

A saturated solution with some amount of the undissolved salt was located on the vessel bottom, while alanine dosimeters in open glass vessels were stored on

the perforated plate. As the volume of the desiccator

was only about 1 L, while the saturated solution surface area was about 100 cm2

, there was no need for

special means to eliminate humidity gradients. An additional desiccator with anhydrous P2O5 was used to

provide a 0% relative humidity environment. Between

measurement sessions, the desiccators were kept sealed

with a silicon grease, and special attention was paid to

keeping the solutions saturated and the phosphorus

pentoxide powder dry at all times. To reduce the

chance of any photochemical eect, the desiccators

with alanine pellets were stored in the dark.

2.2. Dosimeters

In most of the measurements described in this

paper, commercially available Bruker alanine dosimeters (Bruker Instruments: Batch No. 3) have been

used. They contain 80% L-a-alanine and 20% polyethylene and are 4.9 mm in diameter and 5 mm in

height. The dosimeter masses range from 84 to 87 mg.

To simulate the situation most frequently occurring in

practice at present, the dosimeters were used ``as delivered,'' without any preconditioning additional to that

performed by the manufacturer (Bruker Instruments:

55% R. H.; 248C; 3 months). The dosimeters were

irradiated with 60Cog-rays to absorbed doses of 1, 10,

and 100 kGy in a Gammacell (Nordion) with a dose

rate of 9.5 kGy/h under electron equilibrium conditions and controlled temperature (22±248C). The ®rst

signal measurements were performed 2±5 h after the irradiation. The dosimeters were then placed into the

controlled humidity chambers. Three replicate dosimeters were monitored simultaneously for each humidity±dose combination. Before each measurement, the

pellets were removed from the chamber and stored in

the open air in a well-ventilated laboratory for about

1 h. This period was sucient for obtaining a stable,

highly reproducible signal for all of the humidities studied (this eect is explained in Section 4.1).

Some preliminary experiments mentioned below

were performed with L-a-alanine (Aldrich, 99+%) pellets manufactured at NIST. They contain 10% polyethylene (Polysciences, MW=700, 60 mm) as a binder

and are 4.9 mm in diameter and 2.7 mm in height.

They were irradiated as described above and kept

1 The 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 identi®ed are necessarily the best available for this purpose.

O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133 123
Table 2

Reported results of the studies of alanine signal fading at room temperature: experiments with uncontrolled humidity conditions (See footnotes to Table 1)Author/Year Dosimeter

characteristics

Dose(kGy)

Observationperiod(months)

Fading(%)

Number ofmeasurementsessions

Uncertainty(%)

Referencesample

Comments

Randolph and Parrish(1959)

?-a-Ala powder 170 6 < 20 NA NA NA

Bradshaw et al. (1962) ?-a-Ala powder 0.01±1 0.5

00 NA 5 Coal sample, before and

after each ala sample

1 3 5 NA 5 Coal sample, before and

after each ala sample

Bermann et al. (1971) L-a-Ala powder 0.762 24

00 9 2±3 NA

Hansen and Olsen(1985)

L-a-Ala+cellulose

(95+5);D = 4.5 mm,h = 2 mm

< 10 18

00 NA NA NA

500 NA Signi®cant NA NA

Kojima et al. (1986) DL-a-Ala+EPR

(77+33),D = 3 mm;h = 30 mm;2 3 30 mm

1 48 2 Calculated NA NA Calculated from the rate of

signal fading at 80±1208C;no mention of controlledhumidity conditions

Hansen et al. (1987) L-a-Ala+polyvidone

(95+5);D = 4.5 mm;h = 2 mm

< 10 5.5 < 0.5 NA NA NA

700 5.5 12 NA NA NA 10 MeV electrons500 5.5 16 NA NA NA 16 MeV protons1000 5.5 22 NA NA NA Stopping 21 MeV 7Li ions

Olsen et al. (1989) ?-a-Ala+polyvidone

(95+5);D = 4.5 mm;h = 2 mm

0.01±0.1 12

00 NA 1 DPPH, alanine, daily

checks

0.1 48 1.8 NA NA DPPH, alanine, daily

checks

Panta et al. (1989) L-a-Ala powder? 0.01±103 36

00 NA 5 Mn(2+) in MgO, adj.

Kojima and Tanaka(1989)

DL-aAla+polysterene

(70+30);D = 3 mm,h = 30 mm

NA NA ``very stable'' NA NA NA

124 O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133
under ordinary laboratory conditions (20±60% R. H.,

22±258C), unless speci®ed otherwise.

2.3. EPR spectrometry

All measurements with Bruker pellets have been performed on an ESC106 spectrometer (Bruker

Instruments) with a 4103TM cavity at room temperature. The modulation frequency was 50 kHz. The

microwave power was 0.25 mW, the highest within the

linear range of the signal amplitude dependence on the

square root of the microwave power, and the sweep

rate was low enough to make the passage between the

derivative extreme longer than 10 time constants. A

1.8-mT central portion of the alanine spectrum was

measured.

The EPR spectrum of each dosimeter was measured

at two dosimeter orientations diering by approximately 908 with respect to Zeeman ®eld. A high-precision tube, with an inner diameter (5.0 mm) that

closely matched the diameter of the dosimeters (4.9 mm

and an inner fused-quartz pedestal) was permanently

mounted in the cavity. This sample tube provided

reproducible positioning of the dosimeters in the cavity; the dosimeters were inserted and removed with a

pneumatic manipulator. The sample tube remained in

the cavity for the whole duration of the study.

To account for variations in the cavity Q-factor,

which are inevitable in this kind of experiment, an

adjacent ruby reference sample was permanently present in the cavity. It was inserted through the slot for

the illumination grid in the front wall of the cavity.

This design (courtesy of Bruker Instruments) made it

possible to independently adjust the depth of the ruby

crystal insertion into the cavity and the ruby crystal

orientation with respect to Zeeman ®eld, which

allowed both the positions of Cr(3+) spectral lines

and their intensity to be easily changed. The position

and orientation of the ruby crystal selected in the

beginning of the study remained unchanged during the

whole series of measurements. The selected ruby line

was automatically recorded immediately after recording the central line of the alanine spectrum of each

dosimeter at each of its two orientations (settings for

the critical operational parameters of the spectrometer

remained the same). All alanine signal amplitudes

measured in this study have been normalized to the

amplitudes of the ruby line recorded with the alanine

dosimeters at the selected orientations. The ruby-normalized signals have been further normalized to the initial individual masses of the pellets, and the reported

values are an average of three pellets for each dose±

humidity combination (in preparing the plots, the signal intensities have been further normalized to make

all their initial values equal to 100%). The alanine sigTable 2 (

nal of each irradiated dosimeter was measured once or

continued )

Author/Year Dosimeter

characteristics

Dose(kGy)

Observationperiod(months)

Fading(%)

Number ofmeasurementsessions

Uncertainty(%)

Referencesample

Comments

Hansen and Olsen (1989) L-a-Ala+polyvidone

(95+5);D = 4.5 mm,h = 2 mm

``low dose'' 1.4 < 1 2 2.6 Alanine pellet, once a

session

Len (1994) ?-a-Ala+polyvidone

(95+5)

0.1, 10 kGy 2

00 NA NA NA

Mehta (1996) DL-aAla+polystyrene,

(70+30),D = 3 mm,h = 30 mm

15 8 1±1.4 20 0.2±0.3 NA Preconditioned at 50%

RH, 3 irradiationtemperatures (15, 27.5,408C)

DL-aAla+polystyrene,

(70+30),D = 3 mm,h = 30 mm

45 8 1±1.4 20 NA

O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133 125
twice every week during the period of observation. The

measurement of uncertainty is 0.8%.

3. Results

Fig. 1(A±F) shows long-term changes of the EPR

signal of alanine dosimeters irradiated to 1, 10, and

100 kGy and stored in the dark at various constant

humidities. For pellets stored at low and moderate

humidities (Fig. 1A±D), the signal amplitude slightly

increases during the ®rst days after irradiation, and

then begins to decrease steadily. The initial increase in

amplitude of approximately 0.5% has been previously

characterized by Nagy and Desrosiers (1996). The

weak initial rise in signal, however, is not observed in

Figs. 1E and F due to the strong fading (up to 8% at

96 R. H.) induced at high relative humidities.

The fading is relatively slow at low and moderate

humidities. After a 3-month storage period under

``normal'' humidities 33±44%, the signal amplitudes

deviate from their maximal observed values by 1.3, 1.7,

and 02.5% for 1, 10, and 100 kGy, respectively. If one

compares the observed normalized amplitudes with the

amplitudes of not the maximal, but the initial signals,

the decrease in 3 months is below 1% for 1 and 10

kGy, and about 2±2.5% for 100 kGy. At the end of a

1-year storage, the amplitudes for these doses decline

by 4.6, 5.2, and 6.2% of their maximal values, respectively.

Fading rate increases with relative humidity especially at R. H. greater than 60%. At 94% R. H.,

three months after 1 and 10 kGy irradiation, the EPR

signal amplitudes decrease by 13±14% and 18% for

100 kGy. At the end of one year, the decrease is about

20% for 1 and 10 kGy and as large as 25% for 100

kGy.

Even for pellets stored under anhydrous phosphorus

pentoxide, which is one of the strongest dehydrating

reagents known (essentially 0% humidity), the signal

decrease with respect to the maximal value can be

regarded as statistically insigni®cant for 3 months at 1

and 10 kGy, and only 1±2 months for pellets irradiated

to 100 kGy. After one year, the decrease in signal is

about 2% of the initial values for doses of 1 and 10

kGy, and 3.5% for 100 kGy.

Fig. 2 presents the same data in a way convenient

for observing the eect of the dose at a constant

humidity. Also, the expanded scale provides an opportunity to evaluate signal values at various stages of the

storage with a higher precision. Generally, the dierence between the signals of dosimeters irradiated to 1

and 10 kGy is small (in most cases less than 1%) and

is probably statistically insigni®cant.

4. Discussion

Our study of the eect of humidity on the alanine

signal dier from other researchers not only in the

number of measurements, but, more importantly, in

the use of a ruby crystal as a reference sample permanently present in the cavity. The ``external'' reference

samples used by others, such as an irradiated alanine

dosimeter or ``strong pitch'' measured before and after

each measurement series, attempt to monitor and correct for uncontrolled spectrometer sensitivity variations. Even in this capacity they are far from being

perfect: signi®cant changes in the spectrometer sensitivity during a measurement session, which are comparable with the small signal changes due to the

radical decay, are commonly observed. Most important, this type of reference technique provides absolutely no information about the Q-factors of the

cavities loaded with dosimeters, which depend on the

amount of polar substances, such as water, present in

the cavity. The cavity Q-factor directly aects the

amplitudes of recorded spectra; the same number of

paramagnetic centers will produce dierent signal

amplitudes at dierent Q-values, so much so that

methods to determine moisture content of various materials have been proposed that are based on this eect

(Kozlov et al., 1978). Clearly, this eect has to be

properly taken into account in a study where measured

samples are stored at dierent humidities, and water

content of each pellet may change with time. The

changes of Q-factor due to variations in water concentration in dosimeters are signi®cant and must be

characterized independently to measure changes in the

alanine signal amplitude accurately due to radical

decay. Fig. 3 shows the relative values of the ruby

crystal signal from the ®xed reference device and

measured in the presence of dosimeters that have been

stored at various humidities. For storage relative

humidity ranging from 20 to 80%, variations in the

cavity Q-factor due to the dierences in water concentrations in dosimeters may reach 7%. Obviously, these

dependencies cannot be ignored.

Table 3

Relative humidity variations with temperature of saturated

salt solutions used in the study (Rockland, 1960)

Salt

Relative humidity (%C)

158C 208C 258C 308C

Magnesium chloride 33 33 33 32

Potassium carbonate 45 44 43 42

Sodium bromide 58 57 57 57

Sodium chloride 75 75 75 75

Potassium nitrate 95 94 93 92

126 O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133
A ruby crystal permanently present in the cavity

alleviates this problem. Its signal is recorded with the

alanine dosimeter in question present in the same cavity, and the dosimeter moisture content aects the signals of the radicals and Cr(3+) in ruby approximately

to the same extent. Therefore, the alanine signal normalized to the ruby signal becomes independent of

water concentration in the dosimeter.2 Thus, the ruby

Fig. 1. Long-term time dependence of the EPR signal amplitudes of Bruker alanine dosimeters stored at dierent humidities.

2 This is true to the ®rst, but yet, very good approximation.

There are also eects of the microwave ®eld perturbation,

which make signal intensity not exactly proportional to Qvalue (Nagy, 1994), but, in most cases, the deviations from

proportionality are about 2 orders of magnitude smaller than

the changes of signals due to Q-factor variations.

O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133 127
reference sample serves to reliably compensate for both

the variations in the spectrometer sensitivity and the

Q-factor variations that occur due to unpredictable

varying concentration of water in the dosimeters. In

contrast to irradiated-alanine reference samples, the

Cr(3+) signal from ruby is known to be absolutely

stable under normal laboratory conditions. Moreover,

because the crystal is rigidly ®xed, the signal is also

free of the undesirable variations that are observed

with alanine reference pellets due to the slight anisotropy of the alanine signals. Normalizing alanine signals under study to the Cr(3+) signal of a ruby crystal

eectively converts them to a universal scale, making

them free of most temporary in ̄uences and valid for

intercomparison during very long periods of time. The

speci®cs of our experiment, as well as the much larger

body of measurements taken in this study on the

eects of relative humidity, add considerable credence

to the reliability of our data over those previously

reported.

Close analysis of the literature shows that some of

the earlier observations are in reasonable agreement

with our data. In the cases of low and moderate

humidities, the increase in the signal amplitude during

the ®rst days after irradiation is consistent with the

results of our previous studies of the short-term behavior of the alanine signal (Nagy and Desrosiers, 1996);

this eect was also observed in a study by Mehta

(1996). The former study showed that, after a sharp

decrease within the ®rst 2 h after irradiation, the

amplitude of the alanine signal begins to slowly

increase, reaching a maximum within a few days.

Although there are no explicit details on the timing of

measurements in each publication, it is reasonable to

suppose from the reported frequencies of measurements that, in many cases, the ®rst measurement was

taken on the ®rst day after irradiation, while the next

one occurred a considerable time later. This approach

overlooks the period of maximum amplitude and the

actual range of the signal variations turns out to be

underestimated. If we relate the amplitudes of our signals observed over the time not to the maximal, but to

the initial values that occurred on the days of irradiation, the decrease in 100 days for 1 and 10 kGy

at 33 and 44% R. H. will be within 1%, which agrees

with the results of Kojima et al. (1997), Len (1994),

Hansen and Olsen (1989), Van Laere et al. (1989),

Regulla and Dener (1982), and Bradshaw et al.

(1962).

The study performed by Arber and Sharpe (1993)

diers from ours in that a good portion of its focus is

on preconditioning and the eect of changes in storage

humidities. Where comparable, our study is often in

good agreement with the results of that work. For

example, for doses of 1 and 10 kGy, Arber and Sharpe

report similar or even bigger fading degrees for ambiFig. 2. Eect of the irradiation dose on the alanine signal fading at dierent humidities.

128 O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133
ent humidities (40 and 60%) in the case of 1-week

(shortest used) preconditioning. They also point out

that their results demonstrate a stronger fading rate

than was previously believed. However, they measured

smaller fading rates for high humidities than we

observed. Unfortunately, a closer comparison is not

possible because no adjacent reference sample was

used in the published study to monitor cavity Q-factors. Also, data presentation in the form of apparent

doses without a calibration curve hampers exact analysis.

Regulla and Dener (1982) observed 5% fading

over four weeks of storage at 70% R. H. for dosimeters irradiated to 100 kGy. Our data for the same

dose and period of storage at 75% R. H. gives 4.6%

fading. However, to our surprise, only slightly higher

(6.5%) fading was reported by Regulla and Dener at

90% R. H., whereas, according to our observations,

the fading under the same conditions is greater by a

factor of two. Kojima et al. (1992) reported 6% fading

for DL-alanine dosimeters irradiated to 100 kGy and

stored at 50% R. H. for 160 days; this is in very good

agreement with our data for 57% R. H. (6%).

Juncheng and Zaiyong (1996) reported 3% fading in

6.6 months for 0.1 kGy at 58% R. H., our data for 1

kGy and the same humidity and storage period show

4% fading. However, these authors reported fading

much faster than ours for higher humidities and much

slower than ours for the lower humidities.

It is often dicult to strictly compare our results, or

results of others, with the previously published data

due to the lack of experimental details in the publications, the small number of observations taken under

conditions of considerable uncertainties, and dierences in dosimeter composition. Some authors claim

stability considerably higher than the uncertainty of

their measurements.

Careless citations of earlier original papers also contribute to the common but erroneous belief in ``absolute stability'' of the alanine signal. For example, many

authors cite one of the very ®rst papers in alanine dosimetry (Regulla and Dener, 1982) as a proof that the

signal of alanine dosimeters remains constant within

1% over a two-year storage. However, the pellets in

the study by Regulla and Dener were stored at 68C,

which does not correspond to the ``normal laboratory

conditions.'' According to Van't Ho's rule, a decrease

in temperature by 208C usually results in a 4- to 16-

fold decrease in reaction rates. Also, the humidity in

the refrigerator was presumably lower than typical laboratory conditions.

We also have some observations comparing NIST

and Bruker alanine pellets stored under uncontrolled

room humidity. These results are less accurate by de®-

nition, because no adjacent reference sample could be

used in EMS104 EPR spectrometer to monitor the Qfactor and the storage humidity was not controlled.

Nevertheless, the data are of interest since it is the

only direct comparison of humidity eects on pellets of

dierent brands and the measurement conditions are

more similar to those encountered at industrial radiation facilities. These comparisons reveal that NIST

dosimeters demonstrate a signi®cantly (approximately

two times) faster signal fading than Bruker dosimeters.

Because NIST and Bruker pellets dier only slightly in

composition (10% polyethylene in the NIST's pellets

vs 20% in the Bruker's), this discrepancy in behavior

is likely due to the dierence in the pellet shape.

Calculations show that NIST pellets have about 30%

larger surface area per unit mass and, thus, a signi®-

cantly more favorable condition for interface interactions than Bruker pellets. If this hypothesis is

correct, dosimeter shapes that approximate a sphere

(i.e., the shape with the smallest surface/volume ratio)

Fig. 3. Amplitudes of the EPR signal of a ruby reference sample permanently present in the cavity in the presence of alanine dosimeters, which have been stored at various constant humidities for about 1 year.

O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133 129
should have advantages in this respect over more

extended shapes, especially long narrow rods and

®lms, unless the alanine/binder proportions, binder

type, or manufacturing processes are adjusted to compensate for shape dierences. Another interesting observation from this preliminary study is that when

pellets are removed from a humid environment and

placed in a desiccator their signal fading slows down

considerably (approximately ten times slower).

Though semiquantitative, these results demonstrate

that the extensive data provided for Bruker dosimeters

do not necessarily translate to dosimeters of other formulations.

All experiments performed in this study used L-aalanine. It is known that, of the three optical isomers

of a-alanine, this one has the highest activation energy

of the free radical decay (Horan, 1968). Therefore, Da-alanine and DL-a-alanine may actually exhibit an

even faster decay than we observed.

4.1. Practical implications of the dependence of alanine

signal intensity on the cavity Q-factor

The detrimental eect of cavity Q-factor variations

on the alanine signal intensity is an important issue

not only in special studies of long-term fading rates

like the one described above, but also in routine practical dosimetry. Variations in Q-factor on the order of

several percent are not easily noticeable during microwave bridge tuning, but they do produce errors of the

same relative value in measured alanine signals. In

most cases, Q variations result from the diering

moisture content of alanine pellets, which, in turn,

depends on the relative humidity of the storage environment and of the room where the EPR measurements are performed. To determine the doses correctly,

it is important to make sure that either the cavity Qfactor actually remains constant during the whole

series of measurements involving both calibration and

test pellets, or proper corrections for its variations are

applied.

This requirement is widely ignored in practice. Quite

often in measurements, test pellets preconditioned at

room humidity are used in combination with calibration pellets just removed from a desiccator, or

doses to freshly irradiated test pellets are determined

with a calibration curve constructed on a dierent day

with a dierent relative humidity. If no reference

sample is permanently present in the cavity, errors as

large as 5% can occur.

Fig. 4 shows short-term changes in the amplitudes

of EPR signals of alanine pellets measured immediately

after removal from a controlled humidity environment

after storage for months. The curves for absolute alanine amplitudes (square point symbols) re ̄ect what

most EPR alanine dosimetrists deal with in practice. In

the case of pellets just removed from the 0% humidity

desiccator, the alanine signal amplitude noticeably

decreases for at least 5 h, and the decrease during the

®rst two hours is particularly fast. The amplitude of

the ruby signal decreases accordingly; these changes

re ̄ect the Q-factor decrease with time due to the process of absorbing humidity from the laboratory (about

30±40% R. H.). It is worth noting that even in the

case of this moderate relative humidity in the laboratory the signal decreases approximately 1% in 2 h.

If storage humidity is relatively high (44%, 57%,

75%, and 94% R. H.), the absolute amplitude of the

alanine signal increases during the ®rst hours after

entering the EPR laboratory environment; so does the

amplitude of the ruby signal. This re ̄ects the process

of pellet drying in the cavity. The greater the initial

moisture content of the pellet, the stronger is the signal

increase in time. The rate of drying also increases with

modulation amplitude in the 0.28±1.4 mT range,

because higher modulation amplitudes provide higher

temperatures of the pellets during the period of spectrum monitoring.

If the storage humidity (33%) is close to the laboratory humidity, the changes in alanine signal amplitude

are very small.

It should be noted that normalization to the ruby

amplitude keeps the alanine signal constant within

0.2% of its initial value regardless of these drying/wetting processes in all cases except at 94% R. H. In the

latter case, the initial moisture content is so high that

microwave ®eld perturbation eects mentioned above

are very pronounced. This results in a more noticeable

deviation from strict proportionality between the alanine and ruby signal amplitudes, as mentioned earlier.

In this extreme case, ruby normalization does not provide an absolutely perfect signal correction, but, nevertheless, it diminishes the error resulting from Q

changes by as much as a factor of 20. However, the

ruby correction works very well even at relatively high

humidities (75% R. H.).

The magnitudes of the alanine signal changes shown

in Fig. 4 and Table 4 agree well with the data of Fig.

Table 4

Percentage change in the alanine signal amplitude relative to

the initial measurement

Time after removal (h)

Relative humidity (%)

0 33 44 57 75 94

0.5 ÿ0.3 0.15 0.35 0.65 2.1 5.1

1 ÿ0.7 0.2 0.4 0.8 3.3 10.1

2 ÿ1.1 0.25 0.4 0.9 4.1 19.6

130 O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133
3. In comparing them, the reader should bear in mind

that signals in Fig. 3 are normalized to the signal

under ``absolutely dry'' conditions, whereas signals in

Fig. 4 are normalized to the signal at a speci®c storage

humidity.

The data presented above clearly demonstrate that

EPR dosimetrists should take the Q-factor value into

account at all times during their measurements. If no

adjacent reference sample, such as a ruby crystal, is

used, one should ensure that the moisture content of

all the pellets whose signals are to be compared is the

same and stable. The following obvious guidelines

should be followed.

. Test pellets that were preconditioned at an ambient

Fig. 4. Short-term time changes in EPR signal amplitudes of alanine (q), ruby (.), and the alanine-ruby signal ratio (R) recorded

after the removal from the storage environment.

O.F. Sleptchonok et al. / Radiation Physics and Chemistry 57 (2000) 115±133 131
humidity before measurements should not be

directly used in combination with calibration pellets

just removed from a desiccator containing a strong

dehydrating agent.

. Both the test and the calibration vials should contain pellets that were stored together or under identical conditions.

. Test pellets should have moisture content as close as

possible to the calibration pellets on the days of

measurements.

. Irradiated pellets should be preconditioned overnight for moisture content stabilization at the ambient relative humidity of the EPR measurement

laboratory.

. Modulation amplitude should be kept small enough

as not to cause heating sucient to noticeably

change the moisture content of pellets during

measurement.

. One should not overly rely on numerical corrections

for Q-factor changes derived from the experimental

plots like those shown in Fig. 4. (Although these

dependencies characterize the typical situations, we

observed noticeable deviations from these patterns

for some pellets. Ensuring identical experimental

conditions is a more reliable approach than the

introduction of correction factors.)

All these precautions are unnecessary if an adjacent

reference sample is used. A convenient reference

sample reliably ®xed in the cavity, whose non-overlapping signal is recorded immediately before or after the

alanine spectrum with the alanine dosimeter still

hosted in the cavity and minimum number of parameters changed, solves this problem. A high-quality

ruby crystal is particularly good for this purpose,

because a convenient line position can easily be provided just by crystal reorientation, its signal shows no

tendency to saturate at power levels used in alanine

dosimetry, and the signal is very stable. Use of a dual

cavity is less desirable than ®xing a reference sample in

a peripheral area of the conventional cavity, because

the on and o switching of the modulation coils in

two chambers keeps a dual cavity in permanent thermal instability. In conducting very precise measurements of alanine signal evolution with immobile

samples (Nagy and Desrosiers, 1996), good reproducibility was not achievable with a dual cavity.

Arber and Sharpe (1993) concluded that the rate of

the signal fading depends not on the storage humidity

itself, but on its changes. They report that dosimeters

preconditioned at certain humidity for months before

irradiation exhibit much higher stability of the signal

when stored at the same humidity after irradiation.

This might be a very convenient way for handling the

long-term fading problem, but it needs additional veri-

®cation, as the measurement procedure used by those

authors apparently did not involve monitoring the cavity Q-factor. Our data suggest that, in the absence of

an adjacent reference sample, dosimeter preconditioning before EPR measurements is at least as important

as dosimeter preconditioning before irradiation.

5. Conclusions

Our results show that the widespread opinion that

the radiation-induced alanine signal is stable under

normal laboratory conditions for years is incorrect.

The changes in alanine signal are not large enough to

compromise alanine dosimetry as a method, but they

do require attention of dosimetrists. Even at 0%

humidity the signal can be regarded as stable to within

21% of its initial value for not longer than 0.5 year

for doses of 1 and 10 kGy, and only for 3 months for

100 kGy. The rate of fading increases with the relative

humidity, especially above 60%. Fading rates for 1

and 10 kGy at the same humidity are comparable,

while signals of dosimeters irradiated to 100 kGy fade

considerably faster. This pronounced fading should be

taken into account in practice.

Fading characteristics of alanine dosimeters vary

with the shape and possibly the composition of the

dosimeters. Therefore, veri®cation of these eects is

recommended for users when dosimeters are from

dierent sources.

Environmental humidity unfavorably aects the accuracy of EPR dosimetry not only by facilitating the

radical decay, but also by uncontrollably aecting the

cavity Q-factor. Variations in the cavity Q-factor are

easily compensated when an appropriate reference

sample is permanently mounted in the cavity. The

absence of such a reference sample requires that the

EPR dosimetrist implement special precautions and

guidelines.

Noticeable fading of the EPR signal of irradiated

alanine over extended periods of time along with the

complications from the cavity Q-factor instability due

to varying moisture content of the pellets make it

undesirable to use ``standard'' irradiated alanine pellets

for monitoring variations in sensitivity of EPR spectrometers.

It is the recommendation of these authors that the

measurement practices described here should be used

in place of those described in the current ASTM standard on alanine dosimetry (American Society for

Testing and Materials, 1998).

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