Advancements in accuracy of the alanine EPR dosimetry

system

Part III: Usefulness of an adjacent reference sample

Vitaly Nagya,*, Olga F. Sleptchonoka, Marc F. Desrosiers a, Ralph T. Weber b,

Arthur H. Heiss b

a

National Institute of Standards and Technology, Gaithersburg, MD 20899-8460, USA b

Bruker Instruments, Inc, EPR Division, Billerica, MA 01821-3957, USA

Received 26 January 2000; accepted 28 February 2000

Abstract

High stability of the radiation-induced radicals in alanine and the high reproducibility of alanine response to

radiation make it possible to determine radiation doses accurately with Electron Paramagnetic Resonance (EPR)

spectral analysis. Small uncontrollable variations of the EPR spectrometer sensitivity, however, can signi®cantly

deteriorate the accuracy of the method. The errors due to these variations can be eliminated or markedly decreased

if an adjacent reference sample (such as a synthetic ruby crystal) is permanently present in the cavity, and if ratios

of the alanine and ruby signal amplitudes are used throughout the dosimetric session instead of the absolute alanine

amplitudes. This paper addresses methodological aspects of using such adjacent reference samples in alanine

dosimetry and provides illustrations of the usefulness of this technique. 7 2000 Elsevier Science Ltd. All rights

reserved.

Keywords: Alenine dosimetry; EPR; ESR; Adjacent reference sample; Accuracy; Ruby

1. Introduction

The highly reproducible radiation response of alanine measured by Electron Paramagnetic Resonance

(EPR) enables one to evaluate doses with very high accuracy. For example, the overall uncertainty in the

measurement of industrial-range gamma-ray doses

determined with this method at NIST, is currently

speci®ed at only 21.2% (2s). Approximately half of

this uncertainty can be attributed to the EPR spectrometry. Consequently, the technique demands very

high stability in the sensitivity of the EPR spectrometer. In this particular case, the term sensitivity is

de®ned as the signal intensity per absorbing paramagnetic center in a sample with a speci®c composition

and shape measured in a speci®c position and orientation in a speci®c resonant cavity. For accurate dose

assessments, the same sample positioned in a standard

way in the cavity must produce the same signal value

accurate to at least 1%, regardless of the time and day

of the measurements.

This extreme stability is seldom required for most

EPR spectroscopy applications, and this level of stabRadiation Physics and Chemistry 59 (2000) 429±441

0969-806X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 9 - 8 0 6X ( 0 0 ) 0 0 2 7 5 - 9

www.elsevier.com/locate/radphyschem

* Corresponding author. Tel.: +1-301-975-5621; fax: +1-

301-869-7682.

E-mail address: vitaly.nagy@nist.gov (V. Nagy).
ility is seldom achieved in practice. The sensitivity of

EPR spectrometers may vary with time due to a number of factors, some of which are dicult to control.

Among them are  ̄uctuations of the power supply voltages, drifts in the characteristics of the spectrometer

components due to warming or natural aging, variations in the cavity Q-factor, as well as other lessde®ned environmental factors. In addition, some of the

variations (Q-factor changes) are not caused by

changes in the spectrometer response, but by changes

in the properties of the sample such as moisture content. Therefore, special means are necessary for monitoring the spectrometer sensitivity in order to make

accurate corrections for its  ̄uctuations.

An eective technique to solve the problem is to use

the signal of a ®xed adjacent reference sample that is

permanently present in the cavity in a position dierent from that of test samples. Its perfectly stable EPR

signal should not overlap with the signals of the test

samples in order to enable a user to record it with a

test sample still present in the cavity, and the amount

of paramagnetic centers in it does not need to be

known. This technique was ®rst used long ago by

Anderson and Weil (1959), and by Singer (1959). In

the later era of semiconductor-based EPR spectrometers, which are much more stable than their vacuum-tube-based ancestors, the technique was not

frequently used even in quantitative studies. However,

it is necessary to revive this technique to address the

unusually high precision requirements of alanine dosimetry.

The principle of using an adjacent reference sample

is well known. One records its signal before or after

recording the signal of the test sample with the same

or very similar parameters and with the test sample

still present in the cavity. The line(s) of the reference

signal may be within the same scan range as the signal

of the test sample, but not necessarily so. The intensity

of the test sample signal (either the amplitude or the

double integral) is then divided by the amplitude of

the reference sample line and this ratio is used instead

of the absolute value of the test sample signal.

The eect of this technique is two-fold. First,

because the reference signal is recorded along with the

signal of the test sample, the probability of a signi®-

cant change in the spectrometer sensitivity in this short

period of time is very low. Therefore, the ratio of the

two signals is almost independent of the spectrometer

sensitivity and the signal is thus converted into a universal scale that is suitable for intersession comparisons. In this way, eects of power voltage variations,

slow long-term thermal trends, and similar incidental

factors are eectively cancelled out. It should be

emphasized that variations of the spectrometer sensitivity due to the above-mentioned factors can be signi®cant. Variations within 1% are common; for some,

especially older instruments, they can be several times

larger. As a rare example of an unusually unfavorable

case, Fig. 1 shows variations of the EPR signal amplitude of a reference sample recorded by a 10-year-old

Bruker ESP300E 1 spectrometer over a three-month

period. The sample remained undisturbed in the cavity

throughout the observation period; there were considerable periods when the spectra were recorded daily

at 1.5 h intervals. Fig. 1 shows that the signal variations reached several percent in relatively short

periods of time. Of course, such instability is unacceptable for accurate alanine dosimetry.

Second, the adjacent reference sample method is a

good means of eliminating variations in signal intensity

due to a varying cavity Q-factor. Provided that excessive microwave power is not used, the EPR signal

intensity is proportional to the Q-factor. Therefore, the

ratio of the signals is virtually independent of the Qfactor. The main source of Q-factor variations in alanine dosimetry is the dierent and/or changing water

content of dosimeters (Sleptchonok et al., 2000). An

adjacent standard makes it possible to compare signals

of dosimeters with dierent moisture contents directly;

signal changes due to dosimeters with a changing

water content are also corrected for by the standard.

Without this correction, errors in dose measurements

due to the dierences in the water content of dosimeters can easily range from one to several percent.

On one occasion, a commercial dosimeter containing a

Fig. 1. Variations with time of the EPR signal of a stable

paramagnetic material permanently present in the cavity of an

older Bruker ESP300 spectrometer. Signal-to-noise ratio was

greater than 10,000:1. Microwave power: 0.25 mW, modulation amplitude: 0.5 mT.

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 avilable for

this purpose.

430 V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441
metal particle was detected by means of the adjacent

standard technique. The particle dramatically

decreased the Q-factor and consequently the alanine

EPR signal; in the absence of the adjacent reference

sample, the calculated dose would be underestimated

by several tens of percent.

In principle, many paramagnetic materials can be

used as adjacent reference samples. Still, there are

some important requirements. Obviously, the signal of

a reference sample should be absolutely stable and

should not overlap with the signal of the test samples.

Its signal should be suciently strong to allow acquisitions with good signal-to-noise ratios even when

scanning rapidly with a short time constant. The signal

of an adjacent reference sample should respond to

possible small variations in microwave power and

modulation amplitude in the same way as the signal of

the test samples. The most reliable solution is to work

within the linear ranges of the corresponding dependencies of both samples. It must be possible to record

the reference signal with the same or very similar parameters as the signal of the test samples. At the very

least, the modulation amplitude should not be changed

between recordings in order to avoid instabilities arising from thermal expansion and contraction of the

cavity.

Ruby, initially proposed by Singer (1959); Thompson and Waugh (1965), is an ideal material for an

adjacent reference sample. Bruker Instruments EPR

Division has manufactured a prototype accessory using

such a standard. Two views of the prototype that has

been tested at the alanine dosimetry facility at NIST

are shown in Fig. 2. A small (about 5 mm long, 0.4

mm in diameter) synthetic ruby crystal containing

about 10ÿ2

% chromium (3+) is mounted on a plunger

controlled by a precise micrometer mechanism. The

device can be attached to the front wall of a Bruker

ER 4103TM cavity so that the ruby crystal can be

introduced reproducibly into the cavity through the

slots for illumination. This design facilitates the crystal

motion independently along its axis and its rotation

about that axis. Rotation of the crystal makes it possible to get convenient line positions and adjustment of

the depth of the crystal insertion enables one to change

the intensity of the lines as required. Once a convenient crystal orientation is found, it can be locked

reliably in that position. Since both translation and rotation of the sample are independent, incidental

changes in intensities and line position due to rotation

are avoided when adjusting the depth of crystal insertion. A similar construction has been described by

Thompson and Waugh (1965).

2. Methodology

One of the major advantages of ruby as a material

for adjacent reference samples is the high anisotropy

of its signal. This makes it possible to place the relatively narrow reference lines into almost any position

in the spectrum. Ruby is aluminum oxide dilutely substituted with chromium(3+) at aluminum sites. The

chromium ions occupy axially symmetric (trigonal)

sites with the symmetry axis parallel to the crystallographic c-axis. There is no site splitting. The spin

Hamiltonian parameters of the chromium(3+) signal

are: g11 = 1.9817, g_ = 1.9819, and b2

0 = 0.1917

cmÿ1 (Chang et al., 1978). The fairly strong ®ne structure interaction of the three unpaired electrons results

in a highly anisotropic signal. Due to the axial symmetry, the line positions at any crystal orientation are

solely determined by the angle between the externally

applied magnetic ®eld vector and the c-axis of the

single crystal. Fig. 3 demonstrates that many angle

ranges avoid overlap between the chromium and alanine lines. At the same time, there are several possibilities (at angles <108, around 358 and 658) to put a

ruby line as close to the alanine signal as desired if one

Fig. 2. Adjacent reference sample device manufactured by Bruker Instruments. Left: Assembled cavity with the reference sample

device attached and quartz holder for alanine pellets. Right: The same construction in the disassembled form; the dark ruby rod

and the illumination grid used for its insertion can be seen.

V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441 431
wants to record the two signals in the same sweep. It

should be noted that shifts of the lines in the spectrum

due to a change in the crystal orientation are generally

accompanied by changes in their intensity, even though

these changes are not always very noticeable. Hence,

an adjustment of line position in the middle of a

measurement series (if absolutely necessary) generally

requires a careful calibration to establish the appropriate correction factor.

Modern automated spectrometers make it unnecessary to have a reference line in the same spectrum

sweep as the signal of the test sample. An automation

routine can be used to measure the intensity of the alanine and a remote ruby line in separate sequential

scans. Thus, we normally use a line located at 220 mT

at crystal orientation with y 1 288. In order to allow

the spectrometer and signal to stabilize after the ®eld

jump between sequential scans (particularly when

using long time constant), a pre-acquisition delay of a

few seconds is added in the automation routine to

avoid signal distortion.

The dependence of ruby signal intensity on the

depth of the crystal insertion is shown in Fig. 4. The

curve re ̄ects the passage of the crystal through the

area of the strong microwave magnetic ®eld near the

cavity wall. (When a sample is located in the vicinity

of the cavity wall, the signal phase is opposite to the

signal phase in the cavity center due to the sample position with respect to modulation coils). An important

point is that, due to the length of the ruby crystal,

which is extended along the direction of its movement,

the maximum dependence shown in Fig. 4 is not very

sharp. This weak positional dependence, in combination with the high precision of the micrometer

screw, ensures a very good reproducibility of the ruby

signal intensity after crystal repositioning. The relative

standard deviation (RSD) of the replicate amplitudes

was 0.065% for 10 spectrum recordings separated by

the crystal withdrawals/reinsertions, whereas the RSD

of the amplitudes of the signals recorded repeatedly

with immobile ruby crystal was 0.020%. (Both sets of

experiments were performed within a 10-min period on

a Bruker ECS106 with the spectrometer equipped with

a 4103TM cavity.) As RSDs of the EPR signals of an

alanine dosimeter recorded at dierent pellet orientations are usually in the 0.4±0.7% range, this level of

reproducibility is more than sucient for reliably repositioning the ruby standard without a signi®cant contribution to the dose error, if the standard has to be

temporarily withdrawn from the cavity.

It is well known that a favorably strong magnetic

microwave ®eld occurs not only at the wall of the

microwave cavity, but also near its center and, in principle, the ruby crystal could be placed there as well. Its

signal would be even stronger because of the higher

modulation amplitude in that area. However, wall position is superior to the near-central position in two

respects. First, the area with the largest and most

homogeneous microwave magnetic ®eld in the center is

already occupied by the dosimeter. So, it would not be

possible to take advantage of the low ®eld gradient

that provided the broad maximum in Fig. 4 and,

Fig. 3. Dependence of the positions of Cr(3+) lines in the X-band EPR spectrum of ruby single crystal on the angle between the

crystallographic c-axis and the externally applied magnetic ®eld. Frequency: 9.500 GHz, room temperature. Grey rectangles show

crystal orientations at which the alanine signal range is free of ruby lines. (Points of dierent shapes correspond to transitions

between dierent spin states; see Chang et al. (1978) for details).

432 V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441
hence, a good reinsertion reproducibility. Second, the

crystal holder running through the intermediate area

of strong electric microwave ®eld would noticeably

decrease the Q-factor, which may be signi®cant in dealing with weak alanine signals in the case of low doses

or thin alanine ®lms.

As mentioned above, the response of the adjacent

reference sample and the test sample to varying conditions must be the same or very similar in order to

achieve a successful correction. In alanine dosimetry,

one of the main sources of measurement variation is a

change in the eective microwave power incident on

the alanine pellets, which can vary signi®cantly due to

dierent moisture content among the dosimeters. Fig. 5

illustrates this point schematically. Suppose the calibration curve was constructed with dry alanine pellets

that had been stored in a desiccator and the eective

microwave power in them was P1. If the test pellets

were stored at a higher humidity, the eective microwave power in them, P2, will be lower due to a lower

Q value. In the worst-case scenario with no adjacent

reference sample in use (Fig. 5, top), the test pellets

will produce a weaker signal at the same radical concentration (dose). The degree of the decrease will

depend on the shape of the A = f(P1/2) curve, and this

decrease will directly contribute to the error in the

determined dose. Now, suppose that an adjacent reference sample has been used, but its saturation curve is

not proportional to the saturation curve of alanine in

the power range in question (Fig. 5, middle). A shift

from P2 to P1 will cause both the signals to decrease,

but, because the power dependences are not proportional to each other, the correction will not be perfect. The quality of the correction will depend on how

close to proportionality the two curves are. However,

if the measurements are performed in the range of proportionality of the two curves (Fig. 5, bottom), the

correction will theoretically be perfect.

Fig. 6 shows the saturation curves for the ruby and

alanine signals. It can be seen that the ruby signal is

much more dicult to saturate and the upper limit of

the proportionality range (linear in this case) is, thus,

determined by the upper limit of linearity of the alanine response. A careful statistical analysis based on

the comparison of the scatter of replicate signals

recorded at the same power with deviations of the

mean signals of such groups from the linear ®t (Nagy,

2000) demonstrates that the dependence is actually linear only in the 0±0.25 mW range (that is, up to P1/2

= 0.5; this result is also in agreement with the conclusion of Wieser and Girzikowski (1996)). If measurements are performed at a power below this threshold,

the most favorable situation represented in the bottom

portion of Fig. 6 occurs and using the reference signal

should theoretically fully compensate for power variations.

In the high-dose range, which is the main area of application of alanine dosimetry at present, the alanine

signals are very strong and decreasing microwave

power down to 0.25 mW does not deteriorate the signal-to-noise ratio to any noticeable degree. This is also

largely true even for the 10±100 Gy range used in calibrating therapeutical sources. The only area where low

microwave power may be undesirable is dosimetry

with thin alanine ®lms that typically exhibit weaker

signals. However, even in this unfavorable case, high

microwave power is a relatively ineective way to

improve the signal-to-noise ratio because, even in the

linear range, the signal increases only with the square

root of the microwave power.

Fig. 4. Dependence of the amplitude of the ruby line at 220 mT on the readings of the micrometer screw of the ruby holder. Bruker

cavity 4103TM with the standard quartz tube for alanine pellets (O. D. 7 mm; I. D. 5 mm).

V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441 433
Considerations regarding the proportionality of the

two responses are also applicable to  ̄uctuations in

modulation amplitude, although signi®cant variations

of this parameter, apparently, are less common than

variations of the eective microwave power via Q-factor. The highest signal-to-noise ratio for alanine is

achieved at modulation amplitude settings in the range

1.4±1.8 mT, depending on the dose, the microwave

power and the dosimeter shape (the latter determines

the average modulation amplitude over the dosimeter

Fig. 5. Magnitude of the errors due to microwave power variations. Top: No adjacent reference sample used; no correction for signal change as a result of the power decrease from P1 to P2. Middle: An adjacent reference sample is used, but the signals are partly

saturated; a partial correction for the power decrease. Bottom: The power is low enough, so that none of the signals is saturated; a

perfect correction for the power decrease.

434 V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441
shape). However, heating of the cavity and its contents

by the modulation coils is very signi®cant at such high

modulation amplitudes. A dosimeter requires several

minutes to come to thermal equilibrium and thus to

produce a stable EPR signal. Of course, the magnitude

of this eect depends somewhat on the cavity type, design of the dosimeter holder, gas  ̄ow, etc. For our

4103TM cavity with its illumination grid sealed, and

using a quartz tube dosimeter holder with 1-mm-thick

walls and both ends open, the maximum modulation

amplitude providing a stable alanine signal was 0.28

mT.

The amplitude dependence of the ruby line located

at 220 mT at our crystal orientation (linewidth 4.5

mT) on modulation amplitude is linear within the

measurement uncertainties up to 0.6 mT, whereas nonlinearity in the alanine response is already noticeable

at a modulation amplitude of 0.28 mT (Fig. 7). Fitting

the experimental dependence with an analytical function in the range of interest (see Fig. 7), one can calculate the errors that would occur if the modulation

amplitude were to deviate from the preset optimal

value of 0.28 mT (Table 1). Although normalizing the

alanine signal to ruby does not correct the error in full

because of the nonproportionality of the responses, it

decreases the error by approximately a factor of six.

These results are, in fact, for the worst-case scenario,

namely, for the case of the low dose of 0.1 kGy. For

that case, the spin±spin interactions are weak, the spectral line is not broadened signi®cantly and the curvature of the amplitude dependence on modulation

amplitude is, therefore, the largest. For the more commonly measured doses above 10 kGy, which produce

broader spectral lines, the residual error of the rubycorrected signal will actually be smaller. Even though

the responses of the alanine and ruby signals are not

strictly proportional between 0.2 and 0.3 mT, this

range oers optimal results for the following reasons:

the probability of signi®cant variations of modulation

amplitude during measurements is small, partial correction of the resultant errors by means of the ruby

signal is fairly good and using a modulation amplitude

in the alanine linear range would result in about a 10-

fold decrease in the alanine signal from its maximal

value.

Use of an adjacent standard is based on the assumption that the distribution of the microwave ®elds in the

cavity remains unchanged during the measurements.

One of the possible causes of ®eld distribution changes

is rotation of the quartz tube holder containing the

dosimeter in order to obtain spectra at dierent pellet

orientations. For example, at NIST, each pellet is

measured at two mutually perpendicular orientations

with respect to the externally applied magnetic ®eld,

and the dosimeter response is calculated as the mean

of the two ruby-normalized alanine amplitudes. However, no quartz tube holder is absolutely perfect in

terms of the uniformity of wall thickness and concentricity. As a result, the ruby signal slightly varies with

dierent orientations of the holder because the microwave magnetic ®eld at the ruby sample varies. If this

eect is not taken into account, normalization to ruby,

while correcting results for spectrometer instability and

Q-factor variations, will always contribute to the error

in the signal value and, consequently, the dose

Fig. 6. Dependence of the amplitudes of the signals of the

ruby reference sample and alanine on microwave power.

ECS106, cavity 4103TM with the standard quartz tube for

dosimeters; the ruby crystal is located near the front wall.

V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441 435
measurement. In order to minimize this error, one

should investigate the dependence of ruby amplitude

on tube orientation and select orientations that produce approximately the same ruby signal. Results of

such a study for our sample tube are shown in Fig. 8.

The two mutually perpendicular orientations 2 and 8

provide the average dierence in the ruby amplitudes

of only 0.0175%, which is good enough for alanine dosimetry purposes, and these two orientations are now

routinely used in the NIST dosimetry practice. Table 2

lists typical dierences in the ruby signal amplitudes in

the course of a typical dosimetric experiment. As can

be seen, in most cases, the contribution to the total

error from the dierence in the microwave ®eld distribution does not exceed 0.1%, which can be regarded

as negligible in comparison with the contribution from

the alanine signal anisotropy.

An important feature of the device described here is

its suitability for ``single'' cavities. An alternative strategy could be using a dual (H104 mode) cavity with a

reference sample permanently residing in one chamber

and the test samples replaced consecutively in the

other. Although such an arrangement would serve in

the same way as our device in many respects, it has

one signi®cant drawback. One switches from one

chamber to another by turning o one set of moduFig. 7. Dependence of the amplitudes of ruby and alanine signals on modulation amplitude. Least-squares ®t for the alanine dependence in the range 0.05±0.8 mT: Y = 0.474 + 3.877x ÿ 1.252x 2 + 0.000413x/ln x; standard error of the ®t is 0.012.

436 V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441
lation coils and turning on another set of coils. This

keeps the system in a permanent state of thermal

instability and adversely aects the reproducibility of

the results. Our tests have shown that, other conditions

being equal, the reproducibility attainable with a dual

cavity is at least a factor of two worse than the reproducibility that can be achieved with a single cavity.

It is important to note that this paper is devoted to

an intensity and not to a concentration standard. The

number of paramagnetic centers in the adjacent reference sample is not known and even if it were, it could

not be used in quantitations directly, because the reference sample is in a dierent modulation and microwave ®eld than the unknown sample. As the intensity

of an EPR signal is proportional to the modulation

amplitude and to the amplitude of the microwave magnetic ®eld squared, the ratio of signals from these

samples does not represent the ratio of the numbers of

paramagnetic centers in them and cannot be used

directly for quantifying the unknown. A special calibration would be necessary to correct for the dierences in the magnetic ®eld amplitudes. Dierences in

the shapes of the reference and test samples must also

be taken into account (Nagy and Placek, 1992).

3. Bene®ts

We will illustrate the advantages of using the adjaTable 1

Errors in absolute and ruby-normalized alanine signal amplitudes due to deviations in modulation amplitude from the preset optimal value of 0.28 mT. Bruker pellets, dose 100 Gy, microwave power 0.25 mW

Deviation of modulation amplitude (mT) Error without ruby standard correction (%) Error after ruby standard correction (%)

ÿ0.05 ÿ15.6 2.6

ÿ0.04 ÿ12.5 2.1

ÿ0.03 ÿ9.3 1.5

ÿ0.02 ÿ6.2 1.0

ÿ0.01 ÿ3.1 0.5

0.00 0.0 0.0

0.01 3.1 ÿ0.5

0.02 6.1 ÿ1.0

0.03 9.1 ÿ1.5

0.04 12.1 ÿ2.0

0.05 15.0 ÿ2.4

Fig. 8. Dependence of the ruby signal amplitude on the orientation of a quartz tube holder in a 4103TM cavity. Polar

coordinates are used: numbers 1±8 designate eight tube orientations spaced by 458 the radial distance from the center

shows the ruby signal amplitude.

Table 2

Variations in the adjacent ruby standard amplitudes at two

mutually perpendicular orientations of the quartz holder with

an alanine dosimeter

Dosimeter no. Dierence in ruby signals (%)

1 0.02

2 0.01

3 0.13

4 0.10

5 0.01

6 0.08

7 0.04

8 0.02

9 0.01

10 0.06

11 0.01

12 0.10

13 0.07

14 0.01

15 0.08

V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441 437
cent reference sample technique with a few examples.

First, using such a reference makes it possible to detect

and investigate minute changes of signals with time,

which are very dicult to detect otherwise because of

comparable or larger variations in the sensitivity of

EPR spectrometers. Fig. 9 shows typical examples

from our study of alanine signal evolution in the ®rst

days after irradiation. Signal changes are very small

(the range of their variations is usually below 1%) and

they cannot be reliably elucidated or even detected

without an adjacent reference sample (see the top

graphs in Fig. 9). Since the ruby signal recorded immediately before or after each recording of the alanine

signal is aected by the variations of the spectrometer

Fig. 9. Eect of normalization to the signal of the adjacent ruby reference sample in studying evolution of the alanine signal after

60Co gamma irradiation. A 50 kGy; B 100 kGy.

438 V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441
sensitivity approximately in the same way as the alanine signal (middle graphs in Fig. 9), their ratio turns

out to be almost independent of the spectrometer sensitivity and characterizes the real evolution of the alanine signal (the bottom graphs in Fig. 9). It can be

seen from Section A of Fig. 9 that normalizing to the

ruby signal decreases the scatter of points down to one

hundredth of a percent, making it at least an order of

magnitude smaller than it would be without use of an

adjacent reference sample. It was only due to the adjacent reference sample that we were able to investigate

short-term changes of the amplitude of the alanine signal systematically (Nagy and Desrosiers, 1996).

Accurate monitoring of the spectrometer sensitivity

is also very useful in routine dose measurements.

Fig. 10 shows ruby signal amplitudes recorded after

recording the signal of each alanine pellet during a

typical custom source calibration. One can see that a

signi®cant jump in the spectrometer sensitivity

occurred in the middle of constructing the calibration

curve and the sensitivity kept slightly increasing subsequently (Fig. 10a). The signals of the test pellets

(Fig. 10b) were thus recorded under conditions of

enhanced sensitivity. Furthermore, the test pellet doses

corresponded to the part of the calibration curve covered by Pellets 1±9 that were recorded under conFig. 10. Amplitudes of the signals of a ruby adjacent reference sample in the course of dose measurements for a custom source calibration. The pellet numbers show the order of recording the spectra.

Fig. 11. Changes in the amplitudes of alanine and ruby signals, as well as in their ratio, for a dosimeter just brought in from an environment with a relative humidity higher than the humidity of the EPR laboratory.

V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441 439
ditions of ``low-sensitivity'' (Fig. 10a). The adjacent

reference sample corrected for this discrepancy in

recording conditions; if it had not been used, the doses

would be overestimated by about 1%. Normalization

of each alanine signal to the ruby signal made it possible to avoid this signi®cant error. The widely used

technique of checking the spectrometer sensitivity

before or/and after the measurement session by recording a spectrum of a ``standard'' alanine dosimeter cannot make such corrections as accurately as reference

sample signals recorded immediately after recording

the alanine dosimeter signals.

Normalization to ruby helped us to investigate the

temperature dependence of the alanine signal (Nagy et

al., 2000) much more accurately than it was possible

before. Changes in the alanine signal with irradiation

temperature are very small, and the large number of

measurements necessary for statistically signi®cant

results take a long time. During this period, the sensitivity of the spectrometer can vary over a range comparable to or greater than the range of temperaturedependent changes. Normalization to ruby made it

possible to determine the alanine temperature coe-

cients accurately and, in particular, to reveal their nonmonotonic dependence on the dose (Nagy et al., 2000),

which was overlooked in numerous studies before. An

adjacent reference sample is even more important

when one needs to monitor small changes of a signal

over long periods of time, as in studies of long-term

fading of the alanine signal. Using the ruby reference

sample we could consistently measure signals of alanine pellets that were stored under dierent conditions

for a period of about 1 year to study their changes

(Sleptchonok et al., 2000). The measurements of each

signal were performed twice a week and the uncertainty of the measurements was below 1% (2s).

As already mentioned, a very important function of

an adjacent reference sample is to correct dosimeter

signals for variations in the cavity Q-factor. This

aspect is discussed in detail by Sleptchonok et al.

(2000). As a brief illustration, Fig. 11 shows changes in

the signals of alanine and ruby, as well as in their

ratio, for a pellet just removed from an environment

with a relative humidity higher than the relative

humidity in the laboratory. The alanine signal

increases by about 1% due to the dosimeter drying.

Accordingly, so does the amplitude of the ruby signal.

This occurs because both are proportional to the cavity Q-factor, which is dependent on the moisture content in the cavity and the samples. Therefore, the ratio

of the amplitudes remains constant within 0.1%. This

shows that normalizing alanine signals to the ruby signal makes it possible to compare accurately the signals

from pellets of dierent moisture contents, which is

essential for precise dosimetry.

4. Limitations

The above examples demonstrate that using an adjacent reference sample makes it possible to substantially

increase the accuracy of alanine dosimetry in several

ways. However, there are certain restrictions. One of

them is due to the fact that the alanine and reference

signals are not recorded simultaneously, but immediFig. 12. Incomplete correction in the case of a very sharp

change in the spectrometer sensitivity.

440 V. Nagy et al. / Radiation Physics and Chemistry 59 (2000) 429±441
ately one after the other. This means that a very sharp

change in the spectrometer sensitivity may not be corrected in full. Such a situation occurred during the

measurements represented in Fig. 12.

Another limitation is related to microwave ®eld perturbation eects. Corrections for Q-factor variations

are based on the assumption that varying moisture

content of dosimeters changes amplitudes of the microwave ®eld proportionally over the whole cavity. This is

not absolutely correct. Introduction of water (or any

other high dielectric constant material) into a cavity,

also produces changes in the microwave magnetic ®eld

distribution, causing the eect known as microwave

®eld perturbation. The magnitude of the perturbation

eect under conditions of alanine dosimetry is very dif-

®cult to calculate theoretically for a number of

reasons. However, estimations based on theoretical calculations for simpler cases (Nagy, 1994) suggest that

this eect should be very small in comparison with the

errors eliminated by the adjacent standard technique.

In fact, experimental data (Sleptchonok et al., 2000)

demonstrate that normalization to the ruby signal cannot fully correct Q-factor variations only in the

extreme case of measuring the signal of a dosimeter

that just arrived to the EPR lab from an environment

with 94% relative humidity. Under milder conditions,

including 75% relative humidity, the correction was

quite satisfactory.

The third diculty, the reason for which is not quite

clear to us at the moment, is associated with accidental

abnormal shutdowns of the EPR spectrometer due to

power failures or similar mishaps. After the spectrometer is turned on again after such events (even immediately), the ratio of the alanine and ruby

amplitudes may dier from the previous value by as

much as 1%, and it takes the ratio several days to

return to its original value. This phenomenon was

observed two or three times a year on only one of our

spectrometers (ECS106) and may be limited to this

particular machine.

5. Conclusion

The 40-year-old technique of using adjacent reference samples proves to be very useful for alanine dosimetry. It has obvious advantages over the other

reference techniques that are currently utilized and is

recommended for more extensive use in the future.

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