- Email: [email protected]
ESR dosimetry
Calibration Network Based on Alanine/ESR Dosimetry D. REGULLA, A. BARTOLO-ITA,‘” U. DEFFNER, S. ONORI,” M. PANTALONI” and A. WIESER GSF Forschungszentrum fur Umwelt und Gesundheit D-8042 Neuherberg, Germany “Istituto Superiore di Sanita, Laboratorio di Fisica I-00161 Roma, Italy
It is shown that an international network of ESR dosimetry laboratories can exchange alanine samples for precise interlaboratory calibration in the absorbed dose range 0.01-100 kGy. For the experimental verification the alanine/ESR dosimetry laboratories and calibration centers of both the GSF-Forschungszentrum fur Umwelt und Gesundheit, Neuherberg, and the Istituto Superiore di Saniti (ISS), Rome, were mutually acting as a partial network. A blind test, together with the National Physical Laboratory (NPL), Teddington, UK, revealed agreement between the NPL administered doses and the doses reported by GSF and ISS within f 1%.
KEYWORDS: Dosimetry; transfer dosimetry; alanine dosimetry; dosimetry; calibration network. INTRODUCTION
ESR dosimetry;
free radical
The industrial application of ionizing radiation requires absorbed doses between approximately 0.01 and 50 kGy. In this dose range, neither primary dosimetry standards nor sufficient reference dosimetry systems have yet been widely available. The lack of highdose standardization has generally been recognized as a serious problem. The International Atomic Energy Agency (IAEA), Vienna, Austria, has therefore, a decade ago, initiated an International Dose Assurance Service (IDAS) for the member states, which was established as a joint project together with the GSF, Neuherberg, Germany (IAEA, 1981; IAEA, 1984). IDAS was intended to assure world-wide standardization of dose applications within the accepted limits, and dose harmonization in international trade with irradiated products between producers and consumers (Nam, 1988; Nam and Regulla, 1989). Metrologically, IDAS has used a new but meanwhile internationally accepted dosimetry method of high precision. The method is based on concentration measurements of radiation-induced free radicals in the amino acid, alanine, by electron spin resonance (ESR) spectroscopy. It is essential that the free radicals in this organic material remain stable at temperatures up to about 50°C (Bradshaw et al., 1962; Bermann er al., 1971). Chemically, alanine is described by the formula CH,CH(NHJCOOH; the stable free radical results from the loss of the NH2 group. The radiation-induced effect has been established towards precision dosimetry first at the GSF (Regulla and Deffner, 1982), and later at other laboratories, e.g., at the Istituto Superiore di Sanita, Rome (Bartolotta et al., 1984).
Present address: Universita di Palermo, Istituto di Fisica, Via Archirafi 36, I-90123 Palermo, Italy. 23
24
ESR dosimetry and applications
Since the use of IDAS follows a certain administrative protocol, it has not been very flexible. The world-wide increasing activity of industrial radiation processing, however (Nam et al., 1989), as well as the importance of dosimetry as a tool of safety, quality control and quality assurance in this field, depends on an easy access to high-level calibration, preferably on an international basis. Standardized high-level calibration particularly towards the upper limit of the above specified dose range, can be provided by national dosimetry laboratories, or more efficiently by the use of interlaboratory reference dosimetry between secondary standard dosimetry laboratories or specially qualified high-level calibration centers, and the user. The latter concept, as a precondition, needs an appropriate carrier for the dose information, i.e., a transfer dosimeter with reference-class properties. A transfer dosimeter should be characterized by long-lived measuring effect, which should, if possible, be non-destructive by the readout procedure and allows repeated measurements, a sufficiently wide dose range, preferably a linear dose-effect relationship, as well as high precision and reliability. Since the alanine/ESR dosimetry fulfills these criteria to a large extent, an approval for the envisaged interlaboratory transfer dosimetry project was started between GSF and ISS in 1988, which was considered a bilateral approach of a multilateral high-level calibration network. Both Institutes decided to carry out an intercomparison between their ESR spectrometers and alanine dosimeters by using their local MCo calibration facilities for irradiation. The dose range to be covered was chosen to be 0.01 to 100 kGy. The National Physical Laboratory (NPL), Teddington was invited to provide unknown doses apart from calibration doses, and act as a referee for a blind test including both the GSF and ISS alaninc/ESR dosimetry systems. As a basis of the entire research program both laboratories agreed to check carefully their alanine/ESR systems with the aim of optimizing the measurement conditions for both types of alanine samples. The final objective of the program was to prove that alanine dosimeters of different origins allow accurate and reliable dosimetry even when exchanged and evaluated at different laboratories and with different ESR spectrometers. The presently observed worldwide expansion of ESR installations for dosimetry purposes at primary and secondary standard dosimetry laboratories seems to confirm the significance and need for this work (Hansen et al., 1989). EXPERIMENTAL The GSF and ISS alanine dosimeters consist of a blend of commercially available L-alanine with paraffin, both of analytical grade purity. The blend is shaped into solid cylinders (4.9 mm in diameter and 10 mm (GSF) respectively 16 mm (ISS) in length) as obtained by compressing the original powder material in a pill-press. Specifications of the alanine samples are given in Table 1. Recently, the ISS has changed the sample dimensions also to 10 mm length. Table 1. Specifications respective laboratories.
of the GSF and ISS alanine dosimetry
systems as reported by the 1.9s
GSF
Weight ratio alanine/paraffin Diameter, length Specific gravity Linearity of response within 1 % saturation dose Fading* Energy independence of response ESR Spectrometer Only Precision, (95 %**) with one alanine sample Precision, including upside down (95 %**) Interspecimen scattering (95 %**) ESR spectrometer and Precision, alanine dosemeters of a batch (95 %*') overall uncertainty (95 %)***
0.85/0.15 4.9 mm, 10 1.29 gem-3
mm 0.1 Gy-3 0.5 MGy 0.5 % per
o.eo/o.20 4.8 mm, 16 1.16 gCm-3 kGy year
0.2
%
0.2
%
0.8 1.0
% %
1.0 1.8
% %
1.3 2.4
% %
2.3 3.0
% %
* At room temperature, up to 70 % rel. ** confidence level *** Including 2.0 % uncertainty from the reference class dosemeter (95 %**)
humidity calibration
and
mm
In
the
factor
dark of
the
25
ESR dosimetry and applications
For readout of the alanine samples, computer-assisted ESR spectrometers of the types Varian E-l 12 (KS) and Bruker ESP 300 (GSF) were used. Both spectrometers operate in the X-band microwave range. At both Institutes a Dewar-type double-walled quartz sample holder and a standard TE,, cavity were used. All ESR measurements were performed at room temperature. In both laboratories the peak-to-peak amplitude of the first derivative of the absorption spectra was used as a relative measure for the radical concentration. From the total ESR spectrum of alanine with a width of about 13 mT only the amplitude of the central peak was taken for the dose evaluation (Fig. 1). Therefore, the scan range of the magnetic field was restricted to 2.5 mT. Depending on the absorbed dose applied to the alanine samples, the sweep time of the magnetic field was between 30 s and 150s; corresponding time constants were chosen from 0.25 s to 1.25 s. Attempts to use the double integral of the total ESR spectrum, i.e., to determine a relative measure of the free radical concentration for dose evaluation, did not improve the precision. The reason for this is probably the involvement of uncertainty sources, which are difficult to quantify, e.g., the actual width of the ESR spectrum necessary for integration and a non-controlled base-line drift.
I ,
m
a
no
aa4
aal
M8gmUc tIeId, mT
Fig. 1. ESR spectrum of an alanine sample, irradiated with an absorbed dose 1 kGy. The amplitude, h, of the central peak was taken for dose evaluation.
For a linear ESR response of the alanine dosimeters over a wide dose range, the modulation amplitude was selected smaller than the line width and the microwave power below the saturation level of the ESR signal (Figs. 2 and 3). The same settings of the ESR spectrometers could be used for both types of alanine dosimeters, at GSF and ISS. In particular, the modulation amplitude was 0.25 mT and the microwave power was 1 mW, as indicated in Figs. 2 and 3 by the vertical arrows. In order to use the ESR spectrometer over the wide dose range between around 0.01 and 100 kGy, the gain of the spectrometer was adjusted according to the available preset gain factors. The gain factors as given by the manufacturer were in a first approach applied without any further linearity check, Figure 4 shows that the Varian E-l 12 ESR spectrometer needs at least a 30-min warm-up time to reach a constant sensitivity, followed by all readings falling within +0.2 during a 12 hours testing period. As a consequence, the measurements at ISS were performed not before 1 hour after the ESR spectrometer had been switched on. As for the Bruker ESP 300 spectrometer, no warm-up time had to be considered, since all electronic components affecting the spectrometer sensitivity are continuously operating in a stand-by mode. Within a 12-hour stability test, the ESP 300 spectrometer sensitivity was found to be stable within f0.2%, using an irradiated alanine sample for signal generation (1 kGy). The use of alanine samples for reference signal generation appears justified, since ESR technology makes increasingly use of irradiated alanine samples instead of a pitch sample for stability tests. Long-term sensitivity changes of the Varian and Bruker spectrometers were taken into account by regular checks, again using irradiated alanine samples.
ESR dosimetry and applications
1
1
Modulation amplitude. mT
Fig. 2. ESR signal amplitude, h, as a function of the magnetic field modulation amplitude for KS (+) and GSF (0) alanine samples. The arrow indicates the modulation amplitude used for the detector
3
oL 0
i
1
i
(Microwavepowed”.
1
6
mW”
Fig. 3. ESR signal amplitude, h, as a function of microwave amplitude square root of the power for KS (+) and GSF (0) alanine samples. The arrow indicates the microwave power used for the detector
Fig. 4. Increase of the ESR signal amplitude, h, of the alanine samples as a function of time after switch-on of the ESR spectrometer (ISS).
The calibration of both alanine/ESR dosimetry systems in the range 0.01 to 1 kGy was based on secondary standard ionization chamber dosimetry. The beam geometry of a @‘Cotherapy source (Therados from Nordion, Canada, earlier Atomic Energy of Canada Ltd. (AECL)) was used. The GSF calibration in this dose range is traceable to the primary standards of both the National Physical Laboratory (NPL), Teddington, and the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig; the ISS calibration is traceable to the primary standards of the Comitato Nazionale per la Ricerca e per lo Sviluppo dell’Energia Nucleare e delle Energie Alternative (ENEA), Casaccia. In the range 1 to 100 kGy, a @‘CoGammacell irradiator from Nordion, Canada served for calibration, with the dose rate being determined by alanine/ESR dosimetry having been calibrated at the therapy irradiator. The results were cross-checked by Fricke dosimetry, both at GSF and ISS. The absorbed dose rate was approximately 5 kGy/h.
21
ESR dosimetry and applications
For the alanine dosimeters the values of overall uncertainty in Table 1 were obtained by quadratic combination of the component uncertainties, e.g., interspecimen scattering, reproducibility of the reference alanine sample, uncertainty in linearity of the dose-effect relationship and uncertainty of the calibration factors of the reference class dosimeters. Corrections were applied to compensate for the effect of the irradiation temperature on the alanine response, which has a positive temperature coefficient of + 0.0018°C1 (Wieser et al., 1989). RESULTS
The first step of the joint research work was aimed at determining the level of precision achievable at each laboratory in the evaluation of the ESR signal peak heights of the two sources of alanine sample (GSF and ISS). The second step was to check (a) if the dose-effect curve of alanine dosimeters of one laboratory can be used for dose evaluations of alanine dosimeters from another laboratory and (b) if the dose-effect curve can be transferred from a spectrometer A at the laboratory B to a spectrometer C at a laboratory D. As a final step, both laboratories have agreed to carry out a blind test in collaboration with the NPL, Teddington, U.K., to establish the overall uncertainty of the different alanine/ESR dosimetry systems. Reuroducibilitv. The peak-to-peak height, h, of the ESR spectrum (see Fig. 1) was used as a reproducibility test for the two alanine/ESR systems, at GSF and ISS. Within the test, parameters were studied that affect the signal height. The reproducibility test was performed at an absorbed dose of 1 kGy, which is at least three orders of magnitude above the background reading of an unexposed alanine sample. It was found that systematic uncertainties for a given instrument setting are negligible, whereas small random uncertainties were found depending on the instrumental fluctuations as well as on sample properties and on geometrical factors due to sample and cavity. The dependence of h on the position of an irradiated alanine sample in the cavity was studied using fitted quartz spacers (the quartz spacers do not contribute to the ESR signal). Figure 5 shows the values of the ESR signal amplitude, h, as a function of the dosimeter position along the cavity axis, for both the GSF and ISS samples. As expected, h depends less on the displacement of the alanine sample from the cavity center in case of the ISS samples as compared with the GSF samples. This is because the ISS dosimeter, 16 mm long, fills almost the entire effective length of the microwave cavity, and its signal is therefore only slightly influenced by a small vertical displacement. The results also show that the effect of sample displacement from the properly centered position can be minimized and, hence, does not contribute significantly to the overall uncertainty of both dosimeter types.
0.) f -4
-a
-2
0
-1
Dirtrm
from
1
I
J
cwity center. mm
Fig. 5. Relative ESR signal amplitude, h, as a function of the axial distance between the sample and cavity centers (0)
ISS, (x) GSF samples.
2x
ESR dosimetry and applications
As for reproducibility, the ESR signals of the GSF and KS alanine samples were each recorded ten times repeating the sample removal and replacement each time. For this, the respective sample was removed from the microwave cavity and inserted again upside down after some rotation around the sample axis. Additionally all the spectrometer parameters were reset. Under these conditions the signal amplitudes were reproduced within +_1% at a 95% confidence level, for both GSF and ISS alanine dosimeter and ESR spectrometer types. It should be noted that without moving the alanine sample from the cavity the ESR signal amplitudes (see Fig. 1) could reproduced within +0.2%. Interlaboratorv comnarison. 42 GSF and 42 ISS alanine samples were irradiated at several absorbed dose levels between 0.01 and 80 kGy using the irradiation facilities at both laboratories. Three samples were irradiated at each dose level. The mean ratios, hasp/h,,,, and their standard deviation for the alanine samples of the two laboratories, as measured with the GSF and ISS spectrometers, respectively, were found to be l.OS+O.Ol and 1.03&-0.02. The coefficients of variation are, in both cases, within the expected limits, calculated as a quadratic combination of the interspecimen scattering of the two types of dosimeters, assuming that no dependence of dose level exists for the h&h,,, ratio. This hypothesis was checked with the least-square method. The interpolation of the data with a linear function, for instance, resulted into a slope of (2.6+ 1 .0)*10m4for the ISS samples. In both cases, a significance test gave support to the assumption that there is no dose dependence for the h&h,,, ratio. This result shows that, for a given ESR spectrometer, the calibration curve of the GSF and the ISS dosimeters can be obtained from each other with only the need for a suitable instrumental adjustment factor. As a general result, it can therefore be concluded that the dose-effect curve (h in arbitrary units versus absorbed dose) for alanine samples from one laboratory can be used to read similar dosimeters from another laboratory. The vertical scale of the dose-effect curve can easily be adjusted to a different sample sensitivity by the mean value of h for a group of such alanine samples irradiated to a known dose. The number of dosimeters, n, to be used for this adjustment procedure depends on the magnitude of interspecimen scattering, u, for the batch to be evaluated, and on the maximum contribution to the uncertainty, f, to be accepted in the transfer procedure, according to the following relationship, n = (u/f)’ . As for the second step, Fig. 6 shows the dose-effect curve for the GSF alanine samples, obtained with the ISS and the GSF ESR spectrometers; they have identical shape and can be superimposed one to the other with a shift solely in the vertical scale. The same result was found when using alanine samples from ISS.
lcfJ 0.01
.,
0.1
.
.,
. _,
1
lo
Abort&
Dora.
._ 100
ffiv
Fig. 6. ESR signal amplitude, h, of GSF alanine samples as a function of absorbed dose. All samples were irradiated at GSF and measured with the KS (x) and GSF (0) spectrometers.
This shows that the dose-effect curve of alanine dosimeters can be transferred to other laboratories for proper calibration of their ESR spectrometers. One high-quality alanine sample or a group of alanine samples of lower dosimetric quality, irradiated to a reference dose, suffices to adjust the vertical scale.
ESR dosimetry and applications
29
Comoarison with the NPL. A blind test was carried out by GSF and ISS in collaboration with the NPL, which acted as the reference irradiation laboratory. The test was aimed at checking the quality of the alanine/ESR dosimetry system of each laboratory and included a cross-check with the alanine samples of the other laboratory. The reference quantity was the absorbed dose in water. In a first run the GSF and ISS alanine samples were irradiated with @‘Cogamma radiation at NPL, with unknown nominal absorbed doses only. The absorbed doses were chosen between 10 and 100 Gy. These samples were evaluated at GSF and ISS by using their respective calibration facilities. The mean deviation between the evaluated and nominal doses was 1% for the ISS, and 9% for the GSF. The unexpected systematic discrepancy obtained at GSF was caused by an erroneous reference sample. For this reason, the test was repeated. In the second run, the two sets of GSF and ISS alanine dosimeters were brought by courier, to the NPL, to be irradiated once more with YZo gamma rays at absorbed doses between 10 and 100 Gy. For each set, a group of 12 reference dosimeters was irradiated to known absorbed doses to get a comparable calibration basis for GSF and ISS. Another group of 15 samples was irradiated to unknown absorbed doses. The dosimeters were transported back to the reading laboratories by courier again. This procedure was chosen to ensure non-delayed and controlled transport modes; both transports used regular passenger flights. The readout of both dosimeter sets was carried out first at the GSF, two days after irradiation, and second at the ISS ten days later. The absorbed doses as evaluated at GSF and ISS were separately sent to the NPL, which afterwards reported the applied nominal dose values, together with an evaluation of the blind test. Agreement was found between reported and nominal doses within + 1%) whether alanine samples from GSF or ISS were used, or whether the alanine samples were read out at either laboratory. From the results it can also be concluded that for good interlaboratory activities the calibration of the participating alanine/ESR dosimetry systems should best be traceable to one standard. At any rate, the comparable standard deviations obtained from the GSF and ISS alanine dosimeter readings regardless of the place of measurement (Sharpe, 1990) indicate that it is the interspecimen scattering of response of the alanine samples that most affects the overall uncertainty.
CONCLUSION The direct comoarison between the ISS and GSF alanine/ESR dosimetry systems, with the samples irradiated at the respective institute of origin and exchanged between them for readout, reveals that high dosimetric precision can be achieved by choosing appropriate types of alanine samples. It also shows that, independent of the ESR spectrometer, whether or not its quality is sufficient. It is also evident that alanine samples of comparable dosimetric quality can be produced at different laboratories. With a carefully controlled production process, the batch uniformity can be kept at around 0.5-2%. This also holds for the directional dependence of response of the alanine samples. The blind test with the NPL, Teddington, U.K., shows excellent results (Table 2). Nevertheless a small trend was recognized arguing that the dose interpretations by GSF were slightly below the administered values (by about 0.5%), and those of ISS above by about the same percentage (Sharpe, 1990). This disparity was found to be systematic. For explanation it appeared necessary to check the reliability of the gain factor settings. From the findings it became evident that the systematic deviations of the attenuation factors from the nominal values were indeed non-negligible, at least not for the envisaged level of precision for alanine/ESR dosimetry systems and must be taken into account. With the GSF alanine/ESR dosimetry system as an example, the non-linearity of the gain factors reached up to 1.3 %, in the dose range under consideration.
30
ESR dosimetry and applications
Table 2. GSF alanine/ESR dosimetry system: Results of the calibration Physical Laboratory (NPL), Teddington.
DOS. No. Nominal doses D(NPL) (GYI
1989 at the National
Reported doses and ratios D(GSF) D(CSF)/D(NPL)D(CSF)* D(GSF)*/D(NPL) (CY) (GY)
12 13
36.80 36.80 36.80
36.1 36.4 36.6
0.981 0.989 0.995
36.5 36.8 37.0
0.992 1.000 1.005
I4 15 16
41.00 41.00 41.00
40.7 40.7 40.7
0.993 0.993 0.993
41.1 41.1 41.1
1.002 1.002 1.002
17 18 19
45.19 45.19 45.19
44.5 44.a 44.9
0.985 0.991 0.994
44.1 44.9 45.1
0.989 0.994 0.998
20 21 22
57.10 57.10 57.10
56.7 56.9 57.7
0.993 0.996 0.993
57.0 57.2 56.9
0.998 1.002 0.996
23 24 25
07.23 87.23 87.23
86.5 87.6 87.6
0.992 1.004 1.004
86.4 07.5 87.5
0.990 1.003 1.003
11
naan standard deviation
0.993 0.006
0.998 0.005
* Including the corrections for non-linear gain factors
The application of respective corrections to the results of the blind test, clearly overcomes the earlier found small systematic absorbed dose discrepancies between GSF and NPL. The ratios of the corrected GSF to the administered NPL absorbed doses were now within l.OO+O.Ol, with the mean at 0.998 and a coefficient of variation of 0.5%) for the 15 dose evaluations. Similar degrees of agreement were achieved with the alanine/ESR dosimetry system at ISS. The blind test experiments revealed excellent precision which can in principle be achieved with a defined alanine/ESR dosimetry system. This was not unexpected but similar precision can obviously also be achieved with alanine samples prepared or read out at another laboratory. Hence alanine/ESR dosimetry systems are compatible. The check of system compatibility between the GSF and ISS laboratories showed that the necessary calibration curve, in a first approach, can be transferred between different laboratories on the basis of a set of reference alanine samples irradiated to known absorbed doses in the dose range of interest. The calibration curve is thus reconstructed from the readings of the reference samples at each participating laboratory. The individual sensitivity of each ESR spectrometer has, by this method, already been demonstrated. As an alternative, the complete calibration curve can be transferred from one laboratory to another. The curve is typical for alanine material and independent of the physical shape of the samples. To adjust for the sensitivity of the respective ESR spectrometer, only one reference dose is necessary, which can be realized by one accompanying alanine sample of high dosimetric quality or a number of alanine samples, if their interspecimen scattering and directional dependence of response are non-negligible. The results of the present study give proof that a global network of ESR laboratories for intercalibration purposes can be established using alanine/ESR dosimetry. It is the alanine dosimeter that in this context serves as a reliable carrier of high-dose information. The information can be readout repeatedly at different times and places, since it is not affected by the readout process. These properties make alanine/ESR dosimetry superior to thermoluminescence or even ionization chamber dosimetry, for such purposes. The experiences from the GSF and ISS blind test against the NPL dosimetric standards gave additional evidence that alanine reference samples must, for long-term use, be treated carefully, i.e., stored at room temperature or below but above the condensation point of water vapor, i.e., at about
ESR dosimetry and applications
31
lO”C-20°C. Storage should take place in the dark and at moderate relative humidity. The samples should be replaced regularly, e.g., about every 6 months. Finally the production quality of alanine samples was shown to be of great importance for a reliable transfer dosimetry in routine practice. A large interspecimen scattering of response, for instance, needs a larger number of alanine samples for each reference dose to obtain mean reference doses of acceptable statistical significance. The same holds for alanine batches with a non-negligible directional dependence of response of alanine samples. In conclusion, an international laboratory network for purposes of calibration, standardization and harmonization in the high dose application seems feasible.
ACKNOWLEDGEMENT The authors wish to express their gratitude to John Barrett and Peter Sharpe, NPL, Teddington, UK for their kind cooperation in providing the calibration doses and playing the part of referees within the blind test; with their support the investigations demonstrated suitable objectivity.
REFERENCES Bartolotta A., Indovina F.L., Onori S. and Rosati A. (1984) Dosimetry for cobalt-60 gamma-rays with alanine. Radiat. Prot. Dosim., 9, 277-281. Bermann F., de Choudens H. and Descour S. (1971) Application ZIla dosimetrie de la mesure par resonance paramagnetique electronique des radicaux libres creCs dans les acides amines. In: Advances in Physical and Biological Radiation Detectors. IAEA, Vienna, STI/PUB/1269, pp. 31 l-325. Bradshaw W.W., Cadena D.G., Crawford G.W., Jr. and Spetzler H.H.W. (1962) The use of alanine as a solid dosimeter. Radiat. Res., 17, 11-2 1. Hansen J.W., Waligorski M.P.R. and Byrski E. (1989) Inter-comparison of gamma-ray, x-ray, and fast neutron dosimetry using alanine detectors. Radiat. Prot. Dosim., 27, 85-92. IAEA Report (1981) Advisory Group on High-Dose Pilot Intercomparison, IAEA, Vienna. IAEA Report (1984) Research Radiation Processing Dosimetry. IAEA-TECDOC 321, IAEA, Vienna. Nam J.W. (1988) International dose assurance service program of the International Atomic Energy Agency. In: Practical Application of Food Irradiation in Asia and the Pacijk. IAEA-TECDOC 452, IAEA, Vienna, pp. 65. Nam J.W. and Regulla D. (1989) The significance of the International Dose Assurance Service for radiation processing. Appl. Radiat. Isot., 40, 953-955. Regulla D.F. and Deffner U. (1982) Dosimetry by ESR spectroscopy of alanine. Appl. Radiat. Isot., 33, 1101-I 114. Sharpe P. (1990) NPL, private communication. Wieser A., Siegele R. and Regulla D.F. (1989) Influence of the irradiation temperature on the free-radical response of alanine. Appl. Radiat. Isot., 40, 957-959.
It is shown that an international network of ESR dosimetry laboratories can exchange alanine samples for precise interlaboratory calibration in the absorbed dose range 0.01-100 kGy. For the experimental verification the alanine/ESR dosimetry laboratories and calibration centers of both the GSF-Forschungszentrum fur Umwelt und Gesundheit, Neuherberg, and the Istituto Superiore di Saniti (ISS), Rome, were mutually acting as a partial network. A blind test, together with the National Physical Laboratory (NPL), Teddington, UK, revealed agreement between the NPL administered doses and the doses reported by GSF and ISS within f 1%.
KEYWORDS: Dosimetry; transfer dosimetry; alanine dosimetry; dosimetry; calibration network. INTRODUCTION
ESR dosimetry;
free radical
The industrial application of ionizing radiation requires absorbed doses between approximately 0.01 and 50 kGy. In this dose range, neither primary dosimetry standards nor sufficient reference dosimetry systems have yet been widely available. The lack of highdose standardization has generally been recognized as a serious problem. The International Atomic Energy Agency (IAEA), Vienna, Austria, has therefore, a decade ago, initiated an International Dose Assurance Service (IDAS) for the member states, which was established as a joint project together with the GSF, Neuherberg, Germany (IAEA, 1981; IAEA, 1984). IDAS was intended to assure world-wide standardization of dose applications within the accepted limits, and dose harmonization in international trade with irradiated products between producers and consumers (Nam, 1988; Nam and Regulla, 1989). Metrologically, IDAS has used a new but meanwhile internationally accepted dosimetry method of high precision. The method is based on concentration measurements of radiation-induced free radicals in the amino acid, alanine, by electron spin resonance (ESR) spectroscopy. It is essential that the free radicals in this organic material remain stable at temperatures up to about 50°C (Bradshaw et al., 1962; Bermann er al., 1971). Chemically, alanine is described by the formula CH,CH(NHJCOOH; the stable free radical results from the loss of the NH2 group. The radiation-induced effect has been established towards precision dosimetry first at the GSF (Regulla and Deffner, 1982), and later at other laboratories, e.g., at the Istituto Superiore di Sanita, Rome (Bartolotta et al., 1984).
Present address: Universita di Palermo, Istituto di Fisica, Via Archirafi 36, I-90123 Palermo, Italy. 23
24
ESR dosimetry and applications
Since the use of IDAS follows a certain administrative protocol, it has not been very flexible. The world-wide increasing activity of industrial radiation processing, however (Nam et al., 1989), as well as the importance of dosimetry as a tool of safety, quality control and quality assurance in this field, depends on an easy access to high-level calibration, preferably on an international basis. Standardized high-level calibration particularly towards the upper limit of the above specified dose range, can be provided by national dosimetry laboratories, or more efficiently by the use of interlaboratory reference dosimetry between secondary standard dosimetry laboratories or specially qualified high-level calibration centers, and the user. The latter concept, as a precondition, needs an appropriate carrier for the dose information, i.e., a transfer dosimeter with reference-class properties. A transfer dosimeter should be characterized by long-lived measuring effect, which should, if possible, be non-destructive by the readout procedure and allows repeated measurements, a sufficiently wide dose range, preferably a linear dose-effect relationship, as well as high precision and reliability. Since the alanine/ESR dosimetry fulfills these criteria to a large extent, an approval for the envisaged interlaboratory transfer dosimetry project was started between GSF and ISS in 1988, which was considered a bilateral approach of a multilateral high-level calibration network. Both Institutes decided to carry out an intercomparison between their ESR spectrometers and alanine dosimeters by using their local MCo calibration facilities for irradiation. The dose range to be covered was chosen to be 0.01 to 100 kGy. The National Physical Laboratory (NPL), Teddington was invited to provide unknown doses apart from calibration doses, and act as a referee for a blind test including both the GSF and ISS alaninc/ESR dosimetry systems. As a basis of the entire research program both laboratories agreed to check carefully their alanine/ESR systems with the aim of optimizing the measurement conditions for both types of alanine samples. The final objective of the program was to prove that alanine dosimeters of different origins allow accurate and reliable dosimetry even when exchanged and evaluated at different laboratories and with different ESR spectrometers. The presently observed worldwide expansion of ESR installations for dosimetry purposes at primary and secondary standard dosimetry laboratories seems to confirm the significance and need for this work (Hansen et al., 1989). EXPERIMENTAL The GSF and ISS alanine dosimeters consist of a blend of commercially available L-alanine with paraffin, both of analytical grade purity. The blend is shaped into solid cylinders (4.9 mm in diameter and 10 mm (GSF) respectively 16 mm (ISS) in length) as obtained by compressing the original powder material in a pill-press. Specifications of the alanine samples are given in Table 1. Recently, the ISS has changed the sample dimensions also to 10 mm length. Table 1. Specifications respective laboratories.
of the GSF and ISS alanine dosimetry
systems as reported by the 1.9s
GSF
Weight ratio alanine/paraffin Diameter, length Specific gravity Linearity of response within 1 % saturation dose Fading* Energy independence of response ESR Spectrometer Only Precision, (95 %**) with one alanine sample Precision, including upside down (95 %**) Interspecimen scattering (95 %**) ESR spectrometer and Precision, alanine dosemeters of a batch (95 %*') overall uncertainty (95 %)***
0.85/0.15 4.9 mm, 10 1.29 gem-3
mm 0.1 Gy-3 0.5 MGy 0.5 % per
o.eo/o.20 4.8 mm, 16 1.16 gCm-3 kGy year
0.2
%
0.2
%
0.8 1.0
% %
1.0 1.8
% %
1.3 2.4
% %
2.3 3.0
% %
* At room temperature, up to 70 % rel. ** confidence level *** Including 2.0 % uncertainty from the reference class dosemeter (95 %**)
humidity calibration
and
mm
In
the
factor
dark of
the
25
ESR dosimetry and applications
For readout of the alanine samples, computer-assisted ESR spectrometers of the types Varian E-l 12 (KS) and Bruker ESP 300 (GSF) were used. Both spectrometers operate in the X-band microwave range. At both Institutes a Dewar-type double-walled quartz sample holder and a standard TE,, cavity were used. All ESR measurements were performed at room temperature. In both laboratories the peak-to-peak amplitude of the first derivative of the absorption spectra was used as a relative measure for the radical concentration. From the total ESR spectrum of alanine with a width of about 13 mT only the amplitude of the central peak was taken for the dose evaluation (Fig. 1). Therefore, the scan range of the magnetic field was restricted to 2.5 mT. Depending on the absorbed dose applied to the alanine samples, the sweep time of the magnetic field was between 30 s and 150s; corresponding time constants were chosen from 0.25 s to 1.25 s. Attempts to use the double integral of the total ESR spectrum, i.e., to determine a relative measure of the free radical concentration for dose evaluation, did not improve the precision. The reason for this is probably the involvement of uncertainty sources, which are difficult to quantify, e.g., the actual width of the ESR spectrum necessary for integration and a non-controlled base-line drift.
I ,
m
a
no
aa4
aal
M8gmUc tIeId, mT
Fig. 1. ESR spectrum of an alanine sample, irradiated with an absorbed dose 1 kGy. The amplitude, h, of the central peak was taken for dose evaluation.
For a linear ESR response of the alanine dosimeters over a wide dose range, the modulation amplitude was selected smaller than the line width and the microwave power below the saturation level of the ESR signal (Figs. 2 and 3). The same settings of the ESR spectrometers could be used for both types of alanine dosimeters, at GSF and ISS. In particular, the modulation amplitude was 0.25 mT and the microwave power was 1 mW, as indicated in Figs. 2 and 3 by the vertical arrows. In order to use the ESR spectrometer over the wide dose range between around 0.01 and 100 kGy, the gain of the spectrometer was adjusted according to the available preset gain factors. The gain factors as given by the manufacturer were in a first approach applied without any further linearity check, Figure 4 shows that the Varian E-l 12 ESR spectrometer needs at least a 30-min warm-up time to reach a constant sensitivity, followed by all readings falling within +0.2 during a 12 hours testing period. As a consequence, the measurements at ISS were performed not before 1 hour after the ESR spectrometer had been switched on. As for the Bruker ESP 300 spectrometer, no warm-up time had to be considered, since all electronic components affecting the spectrometer sensitivity are continuously operating in a stand-by mode. Within a 12-hour stability test, the ESP 300 spectrometer sensitivity was found to be stable within f0.2%, using an irradiated alanine sample for signal generation (1 kGy). The use of alanine samples for reference signal generation appears justified, since ESR technology makes increasingly use of irradiated alanine samples instead of a pitch sample for stability tests. Long-term sensitivity changes of the Varian and Bruker spectrometers were taken into account by regular checks, again using irradiated alanine samples.
ESR dosimetry and applications
1
1
Modulation amplitude. mT
Fig. 2. ESR signal amplitude, h, as a function of the magnetic field modulation amplitude for KS (+) and GSF (0) alanine samples. The arrow indicates the modulation amplitude used for the detector
3
oL 0
i
1
i
(Microwavepowed”.
1
6
mW”
Fig. 3. ESR signal amplitude, h, as a function of microwave amplitude square root of the power for KS (+) and GSF (0) alanine samples. The arrow indicates the microwave power used for the detector
Fig. 4. Increase of the ESR signal amplitude, h, of the alanine samples as a function of time after switch-on of the ESR spectrometer (ISS).
The calibration of both alanine/ESR dosimetry systems in the range 0.01 to 1 kGy was based on secondary standard ionization chamber dosimetry. The beam geometry of a @‘Cotherapy source (Therados from Nordion, Canada, earlier Atomic Energy of Canada Ltd. (AECL)) was used. The GSF calibration in this dose range is traceable to the primary standards of both the National Physical Laboratory (NPL), Teddington, and the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig; the ISS calibration is traceable to the primary standards of the Comitato Nazionale per la Ricerca e per lo Sviluppo dell’Energia Nucleare e delle Energie Alternative (ENEA), Casaccia. In the range 1 to 100 kGy, a @‘CoGammacell irradiator from Nordion, Canada served for calibration, with the dose rate being determined by alanine/ESR dosimetry having been calibrated at the therapy irradiator. The results were cross-checked by Fricke dosimetry, both at GSF and ISS. The absorbed dose rate was approximately 5 kGy/h.
21
ESR dosimetry and applications
For the alanine dosimeters the values of overall uncertainty in Table 1 were obtained by quadratic combination of the component uncertainties, e.g., interspecimen scattering, reproducibility of the reference alanine sample, uncertainty in linearity of the dose-effect relationship and uncertainty of the calibration factors of the reference class dosimeters. Corrections were applied to compensate for the effect of the irradiation temperature on the alanine response, which has a positive temperature coefficient of + 0.0018°C1 (Wieser et al., 1989). RESULTS
The first step of the joint research work was aimed at determining the level of precision achievable at each laboratory in the evaluation of the ESR signal peak heights of the two sources of alanine sample (GSF and ISS). The second step was to check (a) if the dose-effect curve of alanine dosimeters of one laboratory can be used for dose evaluations of alanine dosimeters from another laboratory and (b) if the dose-effect curve can be transferred from a spectrometer A at the laboratory B to a spectrometer C at a laboratory D. As a final step, both laboratories have agreed to carry out a blind test in collaboration with the NPL, Teddington, U.K., to establish the overall uncertainty of the different alanine/ESR dosimetry systems. Reuroducibilitv. The peak-to-peak height, h, of the ESR spectrum (see Fig. 1) was used as a reproducibility test for the two alanine/ESR systems, at GSF and ISS. Within the test, parameters were studied that affect the signal height. The reproducibility test was performed at an absorbed dose of 1 kGy, which is at least three orders of magnitude above the background reading of an unexposed alanine sample. It was found that systematic uncertainties for a given instrument setting are negligible, whereas small random uncertainties were found depending on the instrumental fluctuations as well as on sample properties and on geometrical factors due to sample and cavity. The dependence of h on the position of an irradiated alanine sample in the cavity was studied using fitted quartz spacers (the quartz spacers do not contribute to the ESR signal). Figure 5 shows the values of the ESR signal amplitude, h, as a function of the dosimeter position along the cavity axis, for both the GSF and ISS samples. As expected, h depends less on the displacement of the alanine sample from the cavity center in case of the ISS samples as compared with the GSF samples. This is because the ISS dosimeter, 16 mm long, fills almost the entire effective length of the microwave cavity, and its signal is therefore only slightly influenced by a small vertical displacement. The results also show that the effect of sample displacement from the properly centered position can be minimized and, hence, does not contribute significantly to the overall uncertainty of both dosimeter types.
0.) f -4
-a
-2
0
-1
Dirtrm
from
1
I
J
cwity center. mm
Fig. 5. Relative ESR signal amplitude, h, as a function of the axial distance between the sample and cavity centers (0)
ISS, (x) GSF samples.
2x
ESR dosimetry and applications
As for reproducibility, the ESR signals of the GSF and KS alanine samples were each recorded ten times repeating the sample removal and replacement each time. For this, the respective sample was removed from the microwave cavity and inserted again upside down after some rotation around the sample axis. Additionally all the spectrometer parameters were reset. Under these conditions the signal amplitudes were reproduced within +_1% at a 95% confidence level, for both GSF and ISS alanine dosimeter and ESR spectrometer types. It should be noted that without moving the alanine sample from the cavity the ESR signal amplitudes (see Fig. 1) could reproduced within +0.2%. Interlaboratorv comnarison. 42 GSF and 42 ISS alanine samples were irradiated at several absorbed dose levels between 0.01 and 80 kGy using the irradiation facilities at both laboratories. Three samples were irradiated at each dose level. The mean ratios, hasp/h,,,, and their standard deviation for the alanine samples of the two laboratories, as measured with the GSF and ISS spectrometers, respectively, were found to be l.OS+O.Ol and 1.03&-0.02. The coefficients of variation are, in both cases, within the expected limits, calculated as a quadratic combination of the interspecimen scattering of the two types of dosimeters, assuming that no dependence of dose level exists for the h&h,,, ratio. This hypothesis was checked with the least-square method. The interpolation of the data with a linear function, for instance, resulted into a slope of (2.6+ 1 .0)*10m4for the ISS samples. In both cases, a significance test gave support to the assumption that there is no dose dependence for the h&h,,, ratio. This result shows that, for a given ESR spectrometer, the calibration curve of the GSF and the ISS dosimeters can be obtained from each other with only the need for a suitable instrumental adjustment factor. As a general result, it can therefore be concluded that the dose-effect curve (h in arbitrary units versus absorbed dose) for alanine samples from one laboratory can be used to read similar dosimeters from another laboratory. The vertical scale of the dose-effect curve can easily be adjusted to a different sample sensitivity by the mean value of h for a group of such alanine samples irradiated to a known dose. The number of dosimeters, n, to be used for this adjustment procedure depends on the magnitude of interspecimen scattering, u, for the batch to be evaluated, and on the maximum contribution to the uncertainty, f, to be accepted in the transfer procedure, according to the following relationship, n = (u/f)’ . As for the second step, Fig. 6 shows the dose-effect curve for the GSF alanine samples, obtained with the ISS and the GSF ESR spectrometers; they have identical shape and can be superimposed one to the other with a shift solely in the vertical scale. The same result was found when using alanine samples from ISS.
lcfJ 0.01
.,
0.1
.
.,
. _,
1
lo
Abort&
Dora.
._ 100
ffiv
Fig. 6. ESR signal amplitude, h, of GSF alanine samples as a function of absorbed dose. All samples were irradiated at GSF and measured with the KS (x) and GSF (0) spectrometers.
This shows that the dose-effect curve of alanine dosimeters can be transferred to other laboratories for proper calibration of their ESR spectrometers. One high-quality alanine sample or a group of alanine samples of lower dosimetric quality, irradiated to a reference dose, suffices to adjust the vertical scale.
ESR dosimetry and applications
29
Comoarison with the NPL. A blind test was carried out by GSF and ISS in collaboration with the NPL, which acted as the reference irradiation laboratory. The test was aimed at checking the quality of the alanine/ESR dosimetry system of each laboratory and included a cross-check with the alanine samples of the other laboratory. The reference quantity was the absorbed dose in water. In a first run the GSF and ISS alanine samples were irradiated with @‘Cogamma radiation at NPL, with unknown nominal absorbed doses only. The absorbed doses were chosen between 10 and 100 Gy. These samples were evaluated at GSF and ISS by using their respective calibration facilities. The mean deviation between the evaluated and nominal doses was 1% for the ISS, and 9% for the GSF. The unexpected systematic discrepancy obtained at GSF was caused by an erroneous reference sample. For this reason, the test was repeated. In the second run, the two sets of GSF and ISS alanine dosimeters were brought by courier, to the NPL, to be irradiated once more with YZo gamma rays at absorbed doses between 10 and 100 Gy. For each set, a group of 12 reference dosimeters was irradiated to known absorbed doses to get a comparable calibration basis for GSF and ISS. Another group of 15 samples was irradiated to unknown absorbed doses. The dosimeters were transported back to the reading laboratories by courier again. This procedure was chosen to ensure non-delayed and controlled transport modes; both transports used regular passenger flights. The readout of both dosimeter sets was carried out first at the GSF, two days after irradiation, and second at the ISS ten days later. The absorbed doses as evaluated at GSF and ISS were separately sent to the NPL, which afterwards reported the applied nominal dose values, together with an evaluation of the blind test. Agreement was found between reported and nominal doses within + 1%) whether alanine samples from GSF or ISS were used, or whether the alanine samples were read out at either laboratory. From the results it can also be concluded that for good interlaboratory activities the calibration of the participating alanine/ESR dosimetry systems should best be traceable to one standard. At any rate, the comparable standard deviations obtained from the GSF and ISS alanine dosimeter readings regardless of the place of measurement (Sharpe, 1990) indicate that it is the interspecimen scattering of response of the alanine samples that most affects the overall uncertainty.
CONCLUSION The direct comoarison between the ISS and GSF alanine/ESR dosimetry systems, with the samples irradiated at the respective institute of origin and exchanged between them for readout, reveals that high dosimetric precision can be achieved by choosing appropriate types of alanine samples. It also shows that, independent of the ESR spectrometer, whether or not its quality is sufficient. It is also evident that alanine samples of comparable dosimetric quality can be produced at different laboratories. With a carefully controlled production process, the batch uniformity can be kept at around 0.5-2%. This also holds for the directional dependence of response of the alanine samples. The blind test with the NPL, Teddington, U.K., shows excellent results (Table 2). Nevertheless a small trend was recognized arguing that the dose interpretations by GSF were slightly below the administered values (by about 0.5%), and those of ISS above by about the same percentage (Sharpe, 1990). This disparity was found to be systematic. For explanation it appeared necessary to check the reliability of the gain factor settings. From the findings it became evident that the systematic deviations of the attenuation factors from the nominal values were indeed non-negligible, at least not for the envisaged level of precision for alanine/ESR dosimetry systems and must be taken into account. With the GSF alanine/ESR dosimetry system as an example, the non-linearity of the gain factors reached up to 1.3 %, in the dose range under consideration.
30
ESR dosimetry and applications
Table 2. GSF alanine/ESR dosimetry system: Results of the calibration Physical Laboratory (NPL), Teddington.
DOS. No. Nominal doses D(NPL) (GYI
1989 at the National
Reported doses and ratios D(GSF) D(CSF)/D(NPL)D(CSF)* D(GSF)*/D(NPL) (CY) (GY)
12 13
36.80 36.80 36.80
36.1 36.4 36.6
0.981 0.989 0.995
36.5 36.8 37.0
0.992 1.000 1.005
I4 15 16
41.00 41.00 41.00
40.7 40.7 40.7
0.993 0.993 0.993
41.1 41.1 41.1
1.002 1.002 1.002
17 18 19
45.19 45.19 45.19
44.5 44.a 44.9
0.985 0.991 0.994
44.1 44.9 45.1
0.989 0.994 0.998
20 21 22
57.10 57.10 57.10
56.7 56.9 57.7
0.993 0.996 0.993
57.0 57.2 56.9
0.998 1.002 0.996
23 24 25
07.23 87.23 87.23
86.5 87.6 87.6
0.992 1.004 1.004
86.4 07.5 87.5
0.990 1.003 1.003
11
naan standard deviation
0.993 0.006
0.998 0.005
* Including the corrections for non-linear gain factors
The application of respective corrections to the results of the blind test, clearly overcomes the earlier found small systematic absorbed dose discrepancies between GSF and NPL. The ratios of the corrected GSF to the administered NPL absorbed doses were now within l.OO+O.Ol, with the mean at 0.998 and a coefficient of variation of 0.5%) for the 15 dose evaluations. Similar degrees of agreement were achieved with the alanine/ESR dosimetry system at ISS. The blind test experiments revealed excellent precision which can in principle be achieved with a defined alanine/ESR dosimetry system. This was not unexpected but similar precision can obviously also be achieved with alanine samples prepared or read out at another laboratory. Hence alanine/ESR dosimetry systems are compatible. The check of system compatibility between the GSF and ISS laboratories showed that the necessary calibration curve, in a first approach, can be transferred between different laboratories on the basis of a set of reference alanine samples irradiated to known absorbed doses in the dose range of interest. The calibration curve is thus reconstructed from the readings of the reference samples at each participating laboratory. The individual sensitivity of each ESR spectrometer has, by this method, already been demonstrated. As an alternative, the complete calibration curve can be transferred from one laboratory to another. The curve is typical for alanine material and independent of the physical shape of the samples. To adjust for the sensitivity of the respective ESR spectrometer, only one reference dose is necessary, which can be realized by one accompanying alanine sample of high dosimetric quality or a number of alanine samples, if their interspecimen scattering and directional dependence of response are non-negligible. The results of the present study give proof that a global network of ESR laboratories for intercalibration purposes can be established using alanine/ESR dosimetry. It is the alanine dosimeter that in this context serves as a reliable carrier of high-dose information. The information can be readout repeatedly at different times and places, since it is not affected by the readout process. These properties make alanine/ESR dosimetry superior to thermoluminescence or even ionization chamber dosimetry, for such purposes. The experiences from the GSF and ISS blind test against the NPL dosimetric standards gave additional evidence that alanine reference samples must, for long-term use, be treated carefully, i.e., stored at room temperature or below but above the condensation point of water vapor, i.e., at about
ESR dosimetry and applications
31
lO”C-20°C. Storage should take place in the dark and at moderate relative humidity. The samples should be replaced regularly, e.g., about every 6 months. Finally the production quality of alanine samples was shown to be of great importance for a reliable transfer dosimetry in routine practice. A large interspecimen scattering of response, for instance, needs a larger number of alanine samples for each reference dose to obtain mean reference doses of acceptable statistical significance. The same holds for alanine batches with a non-negligible directional dependence of response of alanine samples. In conclusion, an international laboratory network for purposes of calibration, standardization and harmonization in the high dose application seems feasible.
ACKNOWLEDGEMENT The authors wish to express their gratitude to John Barrett and Peter Sharpe, NPL, Teddington, UK for their kind cooperation in providing the calibration doses and playing the part of referees within the blind test; with their support the investigations demonstrated suitable objectivity.
REFERENCES Bartolotta A., Indovina F.L., Onori S. and Rosati A. (1984) Dosimetry for cobalt-60 gamma-rays with alanine. Radiat. Prot. Dosim., 9, 277-281. Bermann F., de Choudens H. and Descour S. (1971) Application ZIla dosimetrie de la mesure par resonance paramagnetique electronique des radicaux libres creCs dans les acides amines. In: Advances in Physical and Biological Radiation Detectors. IAEA, Vienna, STI/PUB/1269, pp. 31 l-325. Bradshaw W.W., Cadena D.G., Crawford G.W., Jr. and Spetzler H.H.W. (1962) The use of alanine as a solid dosimeter. Radiat. Res., 17, 11-2 1. Hansen J.W., Waligorski M.P.R. and Byrski E. (1989) Inter-comparison of gamma-ray, x-ray, and fast neutron dosimetry using alanine detectors. Radiat. Prot. Dosim., 27, 85-92. IAEA Report (1981) Advisory Group on High-Dose Pilot Intercomparison, IAEA, Vienna. IAEA Report (1984) Research Radiation Processing Dosimetry. IAEA-TECDOC 321, IAEA, Vienna. Nam J.W. (1988) International dose assurance service program of the International Atomic Energy Agency. In: Practical Application of Food Irradiation in Asia and the Pacijk. IAEA-TECDOC 452, IAEA, Vienna, pp. 65. Nam J.W. and Regulla D. (1989) The significance of the International Dose Assurance Service for radiation processing. Appl. Radiat. Isot., 40, 953-955. Regulla D.F. and Deffner U. (1982) Dosimetry by ESR spectroscopy of alanine. Appl. Radiat. Isot., 33, 1101-I 114. Sharpe P. (1990) NPL, private communication. Wieser A., Siegele R. and Regulla D.F. (1989) Influence of the irradiation temperature on the free-radical response of alanine. Appl. Radiat. Isot., 40, 957-959.