Long-term PTB intercomparison of passive H*(10) dosemeters used in area monitoring

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1 Journal of Instrumentation OPEN ACCESS Long-term PTB intercomparison of passive H*(10) dosemeters used in area monitoring To cite this article: H Dombrowski and S Neumaier View the article online for updates and enhancements. Related content - Recommendations to harmonize European early warning dosimetry network systems H. Dombrowski, M. Bleher, M. De Cort et al. - Detection of rain events in radiological early warning networks with spectrodosimetric systems R. Dbrowski, H. Dombrowski, P. Kessler et al. - Radiation protection and environmental standards Peter Ambrosi Recent citations - Letter to the Editor H. Dombrowski This content was downloaded from IP address on 13/12/2018 at 19:59

2 PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB RECEIVED: February 29, 2012 REVISED: March 22, 2012 ACCEPTED: April 2, 2012 PUBLISHED: April 30, 2012 Long-term PTB intercomparison of passive H*(10) dosemeters used in area monitoring H. Dombrowski 1 and S. Neumaier Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig, Germany harald.dombrowski@ptb.de ABSTRACT: This intercomparison serves to investigate the long-term behaviour of passive (or similarly used active) H*(10) dosemeters which are dedicated to the monitoring of the surroundings of nuclear plants and accelerators and used in the radiation field of the natural ambient radiation. From autumn 2005 to spring 2007, approx. 650 photon and neutron dosemeters were exposed to environmental radiation at three dosimetric reference sites, operated by the Physikalisch-Technische Bundesanstalt (PTB). In addition to the measurements under natural conditions a number of dosemeters was also subjected to an additional irradiation in PTB photon and neutron fields. Ten measuring bodies and institutions, which are involved in ambient monitoring in Germany, Austria and Switzerland, took part in this intercomparison. The intercomparison revealed the typical precision to be expected from long-term dose measurements in the natural environment. The difference of the response of the dosemeters to terrestrial and to cosmic radiation was determined. From the results of this intercomparison some sources of uncertainty were identified and some recommendations to improve area monitoring were derived. KEYWORDS: Dosimetry concepts and apparatus; Radiation monitoring 1 Corresponding author. c 2012 IOP Publishing Ltd and Sissa Medialab srl doi: / /7/04/p04017

3 Contents 1 Introduction 1 2 Methods Ambient radiation dosimetry site (free-field) Cosmic radiation dosimetry site (floating platform) Low-level underground laboratory Handling of the dosemeters 5 3 Reference values Free-field Underground laboratory Irradiation General remarks 8 4 Data evaluation 8 5 Results and discussion Photon dosemeters Neutron dosemeters Long-term behaviour of single detection systems Seasonal behaviour of the detection systems Comparison with PTB requirements for area dosemeters Comparison with IEC standard Comparison with requirements of the REI 17 6 Summary 18 7 Conclusion 19 1 Introduction In order to monitor the radiation in areas surrounding nuclear facilities and accelerators, often passive solid state dosemeters such as thermo-luminescence dosemeters or optically stimulated luminescence dosemeters are used as gamma or neutron detectors. When such dosemeters are exposed at one location over weeks or even months, the following problem arises: The dose contribution of the natural radiation must be subtracted from the total measured value in order to obtain the fraction of the dose which was caused by the nearby facility. Natural ambient radiation, however, is subject to strong temporal and spatial variations. Furthermore, in the course of long-term measurements, the dosimetry system itself can have an influence on the measured value (increase of the indicated 1

4 Table 1. Dosemeters used in this intercomparison. R means type of radiation (ph: photons, n: neutrons). No. Code Dosemeter type Dosemeter model R 1 A1 TLD Harshaw TLD card 7777 in special H*(10) cylinder casing developed by ARCS 2 B TLD ARCS H*(10) area dosemeter TLD-2K-V4 ph 3 C TLD Panasonic TLD-UD814 (Li 2 B // CaSO 4 ) ph 4 D TLD Panasonic TLD-UD802 (like no. 3, but different filtering) 5 E1 Geiger-Müller GammaTRACER Basic ph 6 E2 Geiger-Müller GammaTRACER XL ph 7 F TLD TLD-2K-V4 in cylinder casing developed by ARCS (2 element 100H) 8 G Glass dosemeter OD FGD-203&SC-2 (Asahi Techno Glass) ph 9 H TLD OD-A1203-J92 (material: Al 2 O 3 ) ph 10 I TLD TLD100 / TLD200 (Hershaw) ph 11 K TLD Proprietary development ( 7 LiF TLD in PMMA sphere - /0 35 mm) 12 A2 TLD 2 Harshaw TLD cards 6776 in PE sphere (/0 30 cm) n 13 L1 TLD in moderator TLD 6776 (Harshaw) in PE sphere n 14 L2 TLD in moderator TLD 6776 (Harshaw) in PE sphere and 1 cm lead shield n value due to the inherent activity of the detector material, or decrease of the indicated value due to losses of the dose information by de-excitation of metastable energy levels.). The Directive for the Surveillance of Emission and Immission from Nuclear Installations (REI) [1] sets lower detection limits for the measurement of the annual local dose in Germany. When the REI was compiled it could not be established, however, whether dosimetric systems used routinely are able to keep its requirements when solid state detectors are used. This intercomparison will provide essential contributions to clarifying this issue. Some additional conclusions relevant in the field of dosimetric area monitoring will be drawn. ph ph ph ph 2 Methods The following German, Austrian and Swiss measuring bodies and companies took part in the intercomparison on the basis of a contract between the German Swiss Radiation Protection Association (for the subgroup Dosimetry, Arbeitskreis Dosimetrie ionisierender externer Strahlung, AKD, Engl.: Working Team for the Dosimetry of External Radiation) and the Physikalisch- Technische Bundesanstalt (PTB): Auswertungsstelle des Helmholzzentrums München (formerly 2

5 Figure 1. Photograph of the free-field measuring site, which is the PTB reference measuring site for environmental radiation. GSF-Auswertungsstelle), Dosilab AG, (formerly COMET AG Dosimetry), Gesellschaft für Schwerionenforschung (GSI Darmstadt), Landesanstalt für Personendosimetrie und Strahlenschutzausbildung (LPS), Materialprüfungsamt NRW (MPA NRW), Paul Scherrer Institut (PSI), Seibersdorf Laboratories (formerly ARC Seibersdorf research GmbH), VKTA - FB Sicherheit, Forschungszentrum Karlsruhe - Messstelle für Festkörperdosimetrie and Saphymo GmbH (formerly Genitron Instruments GmbH). The organizer of the intercomparison, PTB, is the national metrology institute of Germany providing scientific and technical services. Different dosemeters of the participants were exposed for typically six months each at three PTB measuring sites, i.e. the free-field reference dosimetry site for environmental radiation, the dosimetry site for cosmic radiation and the low-level underground laboratory for dosimetry and spectrometry (UDO). In addition, some of the dosemeters were irradiated in PTB photon fields. The two-year-long intercomparison, beginning in autumn 2005, was divided into four semesters, two winter and two summer semesters. The tested dosemeters are listed in Table 1. The active instruments E1 and E2 were used like the passive instruments. All measured data as well as the reference values were provided in terms of ambient dose equivalent rate, Ḣ*(10), which is the appropriate quantity for area dosimetry according to EU Directive 96/29 [2] (background information about this quantity can be found in [3]). Reference values were determined by PTB for each single dosemeter (the duration of exposure was slightly different). The reference values were calculated on the basis of monthly average dose values which were derived from data routinely retrieved around the clock at the measuring site for ambient radiation. By combining the results obtained at the sites for ambient and cosmic radiation, it is possible to determine the response of the dosemeters, i.e. the measured dose rate divided 3

6 Figure 2. Photograph of the floating platform, which serves as a measuring site for cosmic radiation. The dosemeters were stored inside the hut. by the reference dose rate, both for the cosmic and for the terrestrial component of the natural radiation [4]. The inherent background was measured by storing some of the dosemeters inside the UDO underground laboratory. The measuring sites are described more in detail in the following. 2.1 Ambient radiation dosimetry site (free-field) To characterize dosemeters with respect to natural ionizing radiation, PTB has established a reference site for the dosimetry of environmental radiation (Figure 1). This measuring site is located in the southern part of the PTB s grounds in Braunschweig at an altitude of 85 m above sea level. The measuring site consists of a flat area (approx. 35 m 55 m) covered with grass and a wooden house with a base of approx. 16 m 2. In particular detectors which are not weather-proof are safely accommodated in the air-conditioned wooden house. This measuring site is used to permanently determine and record the single components of the dose rate (caused by photons, electrons, muons and neutrons) of the natural environmental radiation. For this purpose different instruments have been installed: Photon detectors observe the terrestrial gamma radiation as well as the charged component of the secondary cosmic radiation (SCR). The latter radiation consists of muons and electrons and is in particular detected by the particle detector MUDOS, which is not sensitive to gamma radiation [5]. Neutrons are detected by REM counters based on the process of neutron capture by 3 He. The calibration of the existing detectors for terrestrial radiation and cosmic radiation can be traced back to the primary standards of PTB. 4

7 Long-term measurements allow, for example, the investigation of the influence of precipitation, atmospheric temperature and air pressure on the natural radiation field. This knowledge was used to characterize the radiation detectors of the participants in the field of natural environmental radiation. 2.2 Cosmic radiation dosimetry site (floating platform) A floating platform (Figure 2) has been installed on a lake approx. 15 km away from PTB. The platform consists of rubber pontoons. It bears a hut made of plastic in which detectors can be stored while being weather-protected. The rubber as well as the plastics have a very low content of radionuclides. Because of the water depth of 2.5 m m, the distance from the shore of more than 100 m and the flat area surrounding the lake, the terrestrial component of the environmental radiation is strongly reduced. This is why dosemeters on the platform are exposed almost only to secondary cosmic radiation. This measuring site was, in particular, established to characterize the response of dosemeters with regard to the ambient dose equivalent rate of the secondary cosmic radiation at ground level. 2.3 Low-level underground laboratory The PTB underground laboratory for dosimetry and spectrometry (UDO) was located in the Asse salt mine near Braunschweig at a depth of 490 m below ground 1 [6]. Because of the shielding effect of the rock overburden, in this depth the muon component is suppressed by four orders of magnitude compared to the muon flux at the surface. The materials of the UDO building were selected with respect to a low activity concentration of radionuclides. The very low activity of the rock salt combined with a mean radon level of 60 Bq/m 3 leads to an ambient dose equivalent rate of only 2 nsv/h inside the main measuring room of the laboratory building. It was the laboratory with the lowest radiation level worldwide. It was possible to reduce the dose rate further to 0.1 nsv/h if dosemeters were stored in a lead castle. The underground laboratory was very well suited for the measurement of the inherent background of detectors. In particular, UDO provided a photon calibration facility for calibrations at low dose rates comparable to those in the natural environment and even below. This facility was traceable to primary standards and was unique worldwide. 2.4 Handling of the dosemeters In each semester, the participants tried to ship their dosemeters to PTB nearly simultaneously. The history of every dosemeter (regeneration date, departure and arrival date, etc.) was recorded on a route card. Air transports of dosemeters were avoided, because all articles sent by air freight are x-rayed at the airport. Furthermore, during the flight the photon dosemeters are exposed to a comparably high dose rate which causes a reading induced by charged particles of the cosmic radiation and which cannot be quantified properly by using active transport dosemeters (because these are not built to measure particle radiation). Inside an aircraft, neutron dosemeters would be exposed to a neutron field with a high fluence but with a different energy spectrum compared 1 UDO was operated from 1991 to 2011 in the Asse salt mine. A new underground laboratory, UDO II, will become operational in the Braunschweig-Lüneburg salt mine in

8 Table 2. The number of dosemeters applied in this intercomparison. Period Dosemeter type Free-field Irradiation 1 Irradiation 10 Floating platform UDO Sum Semester 1 Photon (passive) Sphere or active Semester 2 Photon (passive) Sphere or active Semester 3 Photon(passive) Sphere or active Semester 4 Photon (passive) Sphere or active All semesters Any to the neutron energy spectrum at ground level. This could cause a high transport dose and high uncertainties. Some participants arranged a very quick and direct transport of the dosemeters to PTB and back from PTB to keep the transport doses as low as possible. The total dose accumulated by the dosemeters stored in UDO is dominated by the transport dose. A long transport leading to a high transport dose would negatively affect the result. After the dosemeters arrived at PTB, they were immediately taken to the measuring sites. The dosemeters were exposed on the free-field, fixed 1 m above ground on wooden rods. The rods formed a hexagon around an active PTB reference probe. The dosemeters were interchanged weekly by moving the rods (including dosemeters) from one support to the next to make sure that the dosemeters were exposed homogeneously. On the floating platform, the dosemeters were stored in the hut in a hanging position, so that the influence of cosmic radiation on the dose rate was identical to that on the free-field. At UDO, the dosemeters were stored in a lead castle. The lead castle was subdivided into numerous lead trays to ensure that radiation caused by traces of radioactivity in one detector did not affect another detector. Because the resulting dose after half a year of storage is still very low (in the order of 0.5 µsv), the dosemeters were not exchanged between semester 3 and 4 resulting in a total storage time of 1 year. Hence, no data from measurements at UDO from semester 3 exist. In addition, some of the photon and neutron dosemeters exposed on the free-field were irradiated with a dose, which corresponds to a dose accumulated in the natural environment within half a year (irradiation 1), while others were irradiated with a dose ten times higher than this half-year dose (irradiation 10). The gamma irradiations were carried out free in air at a PTB facility using a collimated 137 Cs beam. In semester 2 and 4 the neutron dosemeters were irradiated at the PTB neutron irradiation facility in an uncollimated neutron field produced by a bare 252 Cf source. At the end of each semester, the dosemeters were removed from the measuring sites and sent back to the participants - together with the route cards - the same way they had come to PTB. Table 2 gives an overview of the number of dosemeters evaluated in each semester. The total number of dosemeters applied in this intercomparison was about 630. The moderator spheres for the neutron dosemeters and the active dosemeters were considerably larger than the photon dosemeters. 6

9 3 Reference values 3.1 Free-field All reference dose and dose rate values were calculated independently from the data of the participants by evaluating the measurements of the active PTB detectors on the free-field. The dose rate caused by secondary cosmic radiation cannot be measured directly. The charged component of this radiation, consisting of muons and electrons, was detected by using MUDOS, whose count rate is proportional to the dose rate of the charged component of the SCR. The calibration procedure for MUDOS is described by Wissmann [5]. The neutron radiation was detected by using REM counters based on proportional counters filled with BF 3. The count rate of these detectors was calibrated with respect to Ḣ*(10) by applying the NEMUS spectrometer as a reference instrument [7]. The calculation of the reference values relies on the assumption that the incoming cosmic radiation is identical on the free-field and on the floating platform (which is about 14 km away from the free-field) and that the shape of the spectrum of muons and neutrons is temporally almost constant (though the total particle rates change with time). The environmental photon radiation was detected by employing proportional counters. These instruments do not measure the sum of the cosmic and the terrestrial radiation correctly, because the response to cosmic radiation is much higher than that to photon radiation. Therefore, the independently measured cosmic component is subtracted from the instrument reading of the photon detectors, in a first step, by taking into account the detector response to cosmic radiation and the reference measurements of the SCR (as described above). The result is proportional to the terrestrial component of the environmental radiation. The traceability to the primary standards of PTB is realised by an on-site calibration of the proportional counters with a secondary standard, a Reuter-Stokes high-pressure ionisation chamber of the type RSS131ER, which was calibrated in the primary PTB photon fields. With respect to H*(10), the energy response of this instrument is almost flat in the energy region of interest (300 kev < E < 2.8 MeV), with deviations of ±3 % at maximum. As a final step, the terrestrial and the cosmic dose rate are added up resulting in the total ambient dose equivalent rate Ḣ*(10). 3.2 Underground laboratory At UDO, the dose accumulated inside the lead castle in half a year is in the order of 0.5 µsv. The absolute amount of this value is of minor interest, because this dose is low compared to the transport dose, which was in the order of 10 µsv in some cases. As a consequence, the measured dose rates should be compatible with zero (taking into account the large uncertainty resulting from the fact that the difference of nearly equal numbers is calculated). 3.3 Irradiation For the dosemeters which were irradiated in a 137 Cs photon field, this additional dose had to be added to the environmental dose acquired on the free-field. The data of the active dosemeters were evaluated differently: The response in the photon field was calculated by dividing the measured dose rate by the reference dose rate, while the accumulated dose of the long-term measurements was calculated from the permanently recorded dose rate values and compared with the reference dose. 7

10 Table 3. Typical reference values for photon and neutron dosemeters accumulated in one semester. Dosemeter type Ambient equivalent dose in µsv Free-field Irradiation 1 Irradiation 10 Floating platform Terrestrial radiation Photon Neutron UDO 3.4 General remarks Because the time period of the exposure of various detectors at the measuring sites was not completely identical, reference values had to be calculated for every set of dosemeters (i.e. dosemeters with the same history). Some measuring bodies correct for the transport doses routinely, while others neglect the transport dose. In the case that the transport dose was not subtracted from the measured value, a mean transport dose of 2 µsv per day was subtracted by PTB. This excludes some deviations in the results merely caused by a different procedure. The individually calculated reference values do not only vary from dosemeter set to dosemeter set, but also from semester to semester. The dose applied by artificial additional irradiation was varied in the order of 10 % or 15 %. Rounded reference values are listed in Table 3. The fact that the neutron radiation has its origin in the atmosphere and that no terrestrial neutron radiation exists might lead to the conclusion that the neutron dose rate should be similar at any location, but owing to the different absorption of neutrons in different ground material, e.g. soil or water, this assumption is not true. A major part of the measured dose rate is due to neutrons scattered back from the ground. Depending on the ground material, a difference in the neutron dose rate is found when measurements on the free-field and on the floating platform are compared. Active dosemeters show a ratio of 0.55 between the dose rate on the floating platform and on the free-field. The water around the floating platform absorbs neutrons better than the soil of the free-field, because neutrons are decelerated by the high amount of hydrogen in the water very effectively. Already in the 60s a similar effect was observed [8]: When the backscattering of fast reactor neutrons from soil and water is compared, the dose rate above a water surface is much lower (by a factor of 0.4, if the angle of incidence is 0 ). The results of the cited article cannot be compared with the measured ratio of 0.55 directly, because the cosmic neutron spectrum is different from a reactor spectrum. But this values could be verified by performing a Monte Carlo simulation. 4 Data evaluation The data of the dosemeters were evaluated by the participating institutions according to their standard procedures. Some of the participants made fading corrections. These corrections depend on the detector material and measuring procedures of the participant as well as on the exposure conditions. The results of the dose measurements are not only influenced by the dosemeter type, but also by the methods of readout and data evaluation. The uncertainties shown in the diagrams below only include statistical uncertainties (with a coverage factor of k = 1), which were obtained by 8

11 Figure 3. Response of different dosimetric systems at PTB s reference sites for environmental radiation on the free-field (terrestrial plus cosmic radiation) and on the floating platform (cosmic radiation). The terrestrial response is derived from these data. The results of 4 semesters are plotted in vertical order. The green symbols (on the right-hand side of each figure, where present) denote results of neutron dosemeters; the other data are results of photon dosemeters. calculating the standard deviation of several detectors of one set. If no uncertainty bar is present, the value is based on a single measurement so that a statistical analysis was not possible. 5 Results and discussion The detector responses, i.e. the measured dose values divided by the corresponding reference values, are shown in Figure 3 and Figure 4 grouped by measuring sites and semesters. The measured values are the mean dose values of all detectors of one dosimetry system (exposed under the same conditions). The number in the upper left-hand corner denotes the semester. 9

12 Figure 4. Like Figure 3, but the results of dosemeters which were additionally irradiated are displayed. The figures on the right show the dose rates (different ordinate!) from the zero effect measurement in the underground laboratory. 5.1 Photon dosemeters The data of the photon dosemeters recorded on the free-field comply with the reference values very well. Most of the data of the photon detectors deviate by less than 20 % from the reference value, though tolerances permissible by standard IEC (caused by different influence factors) could lead to much larger deviations [9]. On the other hand, if detectors are exposed on the floating platform, an over-response of all systems is observed. Relative to irradiation 10 data, this over-response lies between 7 % and 35 %. This effect is well known from measurements with active dosemeters in an even more pronounced form [4]. The reactions of the charged particles of the secondary cosmic radiation, high energy muons and electrons, with a dosemeter are different from reactions of photons with a dosemeter, because muons and electrons are directly ionizing particles with a track of almost constant energy loss inside the dosemeter. Hence, muons and 10

13 electrons are always detected when they hit the dosemeter, whereas only a part of the photons is detected (due to absorption in the detector casing, scattering, etc.) resulting in a comparably lower response. Although photon dosemeters are sensitive to both types of radiation they are only calibrated in photon fields! The response of photon detectors to neutrons can be neglected in very good approximation, if the energy spectrum of the neutrons of the cosmic radiation is taken into account (quantitative neutron response data of gas-filled detectors dependent on the neutron energy are e.g. found in [10]). The difference of the dose values recorded on the free-field and on the floating platform is used to calculate the terrestrial response [4]. Because the home calibration of the photon dosimetry systems was done in a way that a combination of terrestrial and cosmic radiation (as observed on the free-field) is on average detected with a response of nearly 1, the over-response to cosmic radiation arithmetically results in a relative under-response to terrestrial radiation. This underresponse is caused by the calibration procedure and not by physical reasons. The uncertainties of the terrestrial response are very high, because the values are based on the difference of two values of similar size. The photon dosemeters which were irradiated in addition to the exposure on the free-field with 137 Cs gamma radiation reproduced the reference values very well: Most of the data points do not deviate from the reference values by more than 15 %. The irradiation 10 results mainly reveal how correct the calibration and fading corrections are. A slight tendency to underestimate the reference values is visible. In contrast to the other diagrams, the measurements made at UDO are displayed differently: Instead of response factors, absolute dose rates are plotted, because plotting response factors for values which are close to zero is not adequate. These measurements brought along the difficulty that a zero value had to be found by subtracting large values from each other: The indicated dose and the transport dose, which should be equal in an ideal case. The huge uncertainty bars are due to this difficulty. Only participants who took special care to reduce the transport dose (by arranging a quick transport, possibly with a shielded box) had a chance to obtain results with a small uncertainty. The dosemeters F and G were transported with a very small and well controlled transport dose, which leads to good results in Figure Neutron dosemeters Contrary to the photon dosemeters, the neutron dosemeters showed an under-response under all conditions (Figure 3 and Figure 4). Influenced by large variations, some data points are by chance located close to their reference value. Because terrestrial neutron radiation does not exist, the diagrams showing the response to terrestrial radiation only show data of photon detectors. The main reason for the under-response can be identified by regarding the results of the irradiations performed in semester 2 and 4: The calibration of the neutron detectors was not exact. It is important to note that the uncertainty of the neutron data is considerably higher than that of the photon data. This fact is caused by the method of detecting neutrons: The neutron dose is calculated by subtracting the dose accumulated in 7 LiF (gamma dose) from that accumulated in 6 LiF (neutron plus gamma dose). This difference is much smaller than the minuend and the subtrahend, so that the uncertainty of the neutron dose is very high. The dose rates to be measured are close to the detection limit. In some cases even negative dose values were found. To obtain more exact values, mean values of several detectors should be calculated. This was done in the case of the data points 11

14 Figure 5. Development of the response of the photon dosimetry systems with time, grouped by measuring sites. A solid black square denotes a change in the dosimetry system analyses after the first semester. Circles and squares with no filling indicate a change in the method of data analyses after the first semester. A2, whereas the data points L1 and L2 are based on only one measurement in most cases. Because only a few data sets are available, no detailed conclusion can be drawn from the neutron data. The more reliable data set A2 shows a response of about 50 % at all measuring sites, but in semester 4 the response factors found after the irradiations were considerably higher (but still smaller than 100 %). This could be due to the fact that in semester 4 the number of irradiated dosemeters was reduced to 2 dosemeters per irradiation, which brings about a very high uncertainty. 5.3 Long-term behaviour of single detection systems Because the intercomparison lasted over two years, the stability of the dosimetry systems could be observed during this time: The development of the response factors of the photon dosemeters during the 4 semesters is displayed in Figure 5. After the first semester some participants changed 12

15 Figure 6. Development of the response of the neutron dosimetry systems with time. The response scale is expanded in comparison with Figure 5. their instruments or altered their method of data evaluation. These cases are marked by large black squares or circles. The graphs illustrate that some measuring systems like K or G produced stable results over all semesters, whereas others show a trend or considerable fluctuations at least in one graph. The data of the active dosemeters E1 and E2 recorded in the environment (free-field and floating platform) show rising response factors during the 4 semesters (effects amounting to 20 % were observed), whereas the results of the irradiations were very stable. This means that the response to photons was almost constant, whereas the response to SCR rose. The reason might be that the energy deposition in the detector by muons is so small that pulses in the region of the lower threshold of the electronics are produced. A small drift of the electronics might therefore change the response of the detector to SCR. Figure 6 illustrates the temporal behaviour of the neutron dosemeters. While system A2 produced rather stable results, systems L1 and L2 failed partly (negative response factors indicate failures of the system) or showed a rising trend of the response. The fact that the response values of system A2 were systematically low could lead to the assumption that the calibration was not correct. Another explanation is based on the fact that the neutron reference values were measured inside a hut on the free-field, while the detectors were fixed outside above the grass. If the fact is considered that water absorbs neutrons much better than dry soil (see section Reference values ), moisture at the surface of the soil could possibly have caused a real difference of the neutron dose rate inside the hut and above the grass. This effect will be examined further in the future. 5.4 Seasonal behaviour of the detection systems One goal of the intercomparison was to find an answer to the question whether of there is a systematic difference between the summer semesters 1 and 3 on the one hand and the winter semesters 2 and 4 on the other hand, because the temperatures and periods of sunshine are higher during summer, which could result in different fading in this season. Some curves in Figure 5 like B or H could suggest such a trend, but taking into account the effect of fluctuations of other curves this observation is uncertain. Therefore, the mean values of all photon dosemeters and all neutron dosemeters were calculated to obtain better statistics. The results of this calculation are plotted in Figure 7 and listed in Table 5 (photon dosimetry systems) and Table 6 (neutron dosimetry systems). Before calculating the mean values, outliers were removed, which were obviously based on flawed measurements (e.g. negative dose values, etc.). These outliers would have disfigured real effects. 13

16 Figure 7. Development of the mean values of all dosimetry systems with time, grouped by photon and neutron dosimetry systems. Table 4. Dates of irradiations 1 and 10 and reference of these dates to the semester. Semester Irradiation (photons) Irradiation (neutrons) Date dd.mm.yyyy Related to period Date Related to period dd.mm.yyyy late middle n/a n/a end end middle n/a n/a beginning middle The mean values of all photon dosemeters were very stable, though the single curves showed large fluctuations. No differences were found between measurements performed in summer and in winter, except for the measurements based on irradiations. This might be surprising because the latter measurements mainly include information about the calibration of the dosimetry system. But it is important to take into account that the results of the irradiated dosemeters were also influenced by fading corrections, which correct for the climatic differences between summer and winter. It is known that the measurements B and H include fading corrections, and indeed the data of both systems showed systematic effects depending on the season (Figure 5). The purely statistical fluctuations of the other systems led to constant mean response values with respect to time. As a hypothesis, the resulting dose values could be influenced by fading in a higher degree, if the irradiation was performed at the beginning of the measuring period. This effect could not be accounted for by the participants, because they did not know the date of the irradiation (Table 4). But if this effect played a significant role, the irradiation 10 response values of semester 2 should be lower than the corresponding values of semester 4, because in semester 2 the irradiation happened at the beginning of the semester, with a high possibility of fading until the end of the semester, while in semester 4, the irradiation happened at the end of the semester. In contrast to this expectation the mean irradiation 10 response values of semester 2 and 4 agree very well (Table 5). Hence, the actual date of the irradiation played a minor role. This observation can be exed by the known fact that 14

17 Table 5. Mean response values of all photon dosimetry systems. Red numbers indicate mean values calculated after removing outliers. Semester Free-field Irradiation 10 Cosmic radiation Terrestrial radiation 1 to Table 6. Mean response values of all neutron dosimetry systems. The numbers in brackets are not directly comparable (see text). Red numbers indicate mean values calculated after removing outliers. Semester Free-field Irradiation 10 Cosmic radiation n/a (0.59) 0.63 n/a n/a (0.71) 0.50 n/a 1 to n/a Terrestrial radiation exponential functions describe the process of fading well [11]. Therefore, fading within the first days after exposure is very high, whereas the influence of this effect decreases after long periods. Another important observation is that the mean dose values of all photon dosemeters obtained on the free-field (terrestrial + cosmic radiation) almost perfectly agree with the PTB free-field reference values, while the cosmic radiation was detected with an over-response and the terrestrial radiation was consequently detected with an under-response (Table 5). The under-response of the dosemeters to terrestrial radiation agrees with the results of the irradiation in a PTB photon field: The mean response factors of both types of measurement agree very well, especially if the fact is taken into account that the measurements of the first semester cannot entirely be compared with the other measurements because of changes of instruments and methods after the first semester. As a consequence the conclusion is drawn that the mean calibration factor of the systems is almost about 10 % too low compared with PTB reference values. Because of the over-response to cosmic radiation, the instruments should also show an over-response of the free-field data, if they are calibrated correctly in a photon field. This discussion is not valid for systems E1 and E2, as these are active systems and are not affected by fading. 15

18 The corresponding discussion of the neutron data (Table 6) is simpler because there is no neutron radiation of terrestrial origin. Indeed, the result of the measurements on the free-field and on the floating platform agree widely as expected. These values were not as stable over time as the corresponding values of the photon dosemeters. The fact that these values are systematically too low was partly discussed above (last paragraph of section 5.3). The irradiation 10 measurements should give a hint as to whether the calibration of the systems was correct. The related mean response values of semester 2 are only based on the data of systems L1 and L2, while the value of semester 4 also includes the high response value 0.90 of system A2. Hence, these mean values have no statistical significance. The operator of systems L1 and L2 indeed reported that he found that his calibration factors were too low, while the operator of A1 could not find such an error. The idea that the deviations of the measured values from the reference values could also be caused by a real physical effect, the moisture of the lawn, was discussed above. 5.5 Comparison with PTB requirements for area dosemeters The measured data can be used to obtain an idea as to whether the properties of the tested photon dosimetry systems are in agreement with some of the PTB type approval requirements for area photon dosemeters. In spite of the fact that the PTB requirements are related to laboratory conditions and that they do not cover the absolute calibration, it is possible to check whether the measured data would violate the following requirements under real out-door measuring conditions: The linearity can be checked by applying the following PTB criterion to the free-field data, irradiation 1 data and irradiation 10 data (listed with increasing dose rate) of all measuring periods: (A max A min )/(A max + A min ) < 0.1 (A max is the maximum value of the response, A min is the minimum value of the response). In all cases this criterion is fulfilled, the linearity is even better than 0.05 in most cases. The variation coefficient can be calculated from the data displayed in Figure 5 by evaluating the variation of the response in the four semesters. In the dose range covered by this intercomparison the variation coefficient has to be smaller than 5 %. Also this criterion is fulfilled by systems G and K. The variation coefficient of systems A1, B, E2 and F is slightly larger (up to 8 %). Systems C, D and I showed the highest variation coefficients up to 15 % if all measurements displayed in Figure 5 are considered. It has to be noted that the PTB requirement is related to the short-term stability, while the evaluation made here reveals information about the long-term stability. Details about practical aspects of dosimetry with passive detectors are found in the German standard DIN [12]. It points out that the measured results can vary to a larger extent if longer measuring periods are considered: The admissible shift of the response of a system is limited to 5 % within a time period of only 8 h. The response to energy can be checked within a limited frame. The mean energy of the natural spectrum on the free-field (1.2 MeV) is higher than the mean energy of the spectrum of irradiation 1 (about 0.93 MeV, free-field plus 137 Cs irradiation) and this again is higher than the mean energy of irradiation 10 (about 0.7 MeV). The corresponding PTB requirement is: (A/A 0 ) - 1 < 0.4 (A is any measured response value of a system and A 0 the response value 16

19 under reference conditions). This criterion is fulfilled by all systems in every semester. Even values < 0.1 are reached in most of the cases (exceptions: one value of each system D and I). In conclusion, the measurements of this intercomparison did not show that PTB type approval requirements were violated. To prove that systems fully comply with the PTB requirements would need many more investigations. 5.6 Comparison with IEC standard In the standard IEC , entitled Radiation protection instrumentation Passive integrating dosimetry systems for environmental and personal monitoring [9], requirements similar to those in the PTB type approval requirements are defined. The requirements considered in the following are also compatible with those of standard DIN (Environmental monitoring using integrating solid-state dosimeters) [12]. In this section, the same approach is adopted as that used in section 5.5: The relative response due to non-linearity shall be 5 % at maximum in the dose range from 0.5 msv to 20 msv. Below and above this dose range a higher tolerance (between -9 % and +11 %) is admissible. According to the results of section 5.5 even the 5 % limit is adhered to in most cases. Only systems F and I showed non-linearities up to 10 %. System F has the highest response to cosmic radiation (about 1.4). This effect raises the free-field response values considerably (by 12 %) compared to the response values found in pure photon fields. Hence, the deviations do not clearly show a nonlinearity. But the free-field dose values lie in the region where a higher tolerance is admissible. The values of system I show high fluctuations because of poor statistics, as mentioned above. Also limits of the variation coefficient are defined slightly more strictly in IEC than in the PTB requirements: In the dose range from 0.5 msv to 20 msv the variation coefficient shall be smaller than 3 % and smaller than 5 % otherwise. These requirements are nearly fulfilled by the tested systems G and K, while the other systems show higher variation coefficients, as described in section 5.5. The relative response due to the mean photon radiation energy, combined with the angle of incidence, shall be in the range between -29 % and +67 % (related to the response under reference conditions). Even if the angle of incidence is not taken into account (because it did not play any role in this intercomparison), all systems showed a performance which was still much better in almost all cases. If possible additional tolerances for effects due to the temperature and light exposure are considered, which could affect the long-term stability, the systems under test are in general compatible or almost compatible with IEC standard Comparison with requirements of the REI According to the REI, the German Directive for the Surveillance of Emission and Immission from Nuclear Installations [1], the lower detection limit for the H*(10) measurement a) of direct photon radiation is 0.1 msv per year and b) of direct neutron radiation is 0.5 msv per year (whereby an error in appendix A of this directive is interpreted in the light of appendices B and 17

20 C). Because area dose values in terms of ambient dose equivalent are used to roughly estimate the effective dose, the detection limits of the dose values related to one semester should be 0.05 msv (photon radiation) and 0.25 msv (neutron radiation). The deviations of the absolute free-field photon data (natural environmental radiation) were much larger: In a first step, the absolute variations of the data measured with different dosimetry systems are compared (because a list of all data and individual reference data would be out of the scope of this article, the reader could deduce absolute values approximately from Figure 3 by multiplying the response values with the mean free-field dose of msv). In semester 1 dose values between msv and msv were measured. But this range of values is compatible with PTB type approval requirements and IEC 62387, though the REI does not require a type approval. In a second step, the stability of single systems is studied. It has to be so good that an artificial dose contribution of 0.05 msv can be clearly detected. That means that the variations caused by uncertainties should be smaller than at least msv when the absolute half-year free-field values are compared (as proposed in the preceding paragraph, the reader could deduce absolute values approximately from Figure 5 by multiplying the response values with the mean free-field dose of msv). Only system K showed a much better stability < 0.01 msv, the result of systems A1, F and G was < 0.03 msv, while the other systems showed much larger deviations up to 0.11 msv. This means that many systems have problems in detecting small artificial dose rates in the natural environment because of fluctuations in the measured values (high variation coefficients). The ability of both neutron dosimetry systems to detect an additional artificial contribution to the total dose was shown by performing irradiations. But the absolute values disagreed by a factor of 2 in several cases, because one of the systems showed large fluctuations, while the other systems were stable and delivered reproducible results. In addition, the problem has to be solved that all absolute values were too low by a factor of 0.5 or 0.6 compared with the reference values. This aspect was discussed above in section Summary To summarize the results of the AKD-PTB intercomparison, photon dosimetry by means of passive dosemeters yielded good results of absolute dose measurements, whereas it became apparent that there is still a need for clarification in the area of neutron dosimetry. The results of the intercomparison have contributed to improving measurement procedures for the surveillance of nuclear plants or accelerator facilities. In general, the home calibration of the photon dosimetry systems led to response factors in the region between 80 % and 120 % of the reference value (with some outliers). This source of deviation of the absolute dose values could be removed by a better calibration. The long-term stability of systems G and K was very high on all measuring sites, if the overall variation coefficient is calculated, and the stability of systems E2 and F was high, under the restriction that E2 showed a slight trend. But some systems showed a pronounced tendency (rising or falling) or drastic changes of their response factors of about 20 % during the 4 semesters. This lack in stability causes problems in fulfilling the requirements of the German Directive for the Surveillance of Emission and Immission from Nuclear Installations (REI), which defines absolute detection limits. One of the neutron dosimetry systems showed a high stability and reproducibility. However, all measured 18

21 neutron dose values of this system understated the reference values systematically by 40 %. This might be caused by the fact that the humidity of the soil was not taken into account. This effect has to be investigated further. All photon detection systems showed an over-response to secondary cosmic radiation, which can reach 40 %. This effect has to be taken into account if measurements in the natural environment are compared with reference values. The systems should be calibrated in photon fields, preferably in the gamma radiation of a sealed 226 Ra source (which resembles natural terrestrial radiation very much), so that additional gamma radiation can be quantified correctly in the natural environment. As a consequence, an overestimation of the total dose rate (including cosmic radiation) up to 20 % (depending on the dosimetry system) cannot be avoided. The two active systems integrated in this intercomparison showed similar properties as the passive systems, but their response to cosmic radiation rose during the intercomparison probably because of the drift of the electronics. The advantage of these systems is that their data storage contains information about the dose rate history and that their data do not have to be corrected because of fading. Fading corrections of the data of passive systems cause problems when the measuring periods are very long: When dosemeters were irradiated artificially in addition to the environmental exposure, the need for fading corrections caused additional deviations from the reference values. 7 Conclusion Though the measurements of all systems performed in the context of this intercomparison neither violate the type approval requirements of PTB nor requirements by IEC in general, the systems have difficulties in fulfilling the requirements of the REI because of the limited sensitivity and stability of the systems. Under identical conditions, systems which fully comply with the PTB type approval requirements can indicate absolute values which differ by a factor of 2 or even 3. This is caused by a combination of several permitted tolerances on the one hand (each tolerance can amount to some 10 % in the field of area dosimetry) and by deviations of the absolute calibration from reference values or instabilities of the calibration on the other hand. Against the background of these potential sources of discrepancy the results of the photon dosimetry systems were in surprisingly good agreement with the reference values (deviations were smaller than 20 % in most cases). But the observed fluctuations of most systems lead to the conclusion that measuring periods longer than 3 months are not recommended in area monitoring. Nevertheless, the differences of the results of some measurements at the same place under the same conditions, which might differ by more than 50 % absolutely, are not satisfying. In area dosimetry and radiation monitoring a better agreement of measurements of different systems and dosimetry services is highly desirable. The participants reported that this intercomparison was an important contribution to their quality assurance system. Therefore, regular intercomparisons are needed to harmonize the calibration factors on the basis of the PTB reference values traceable to the primary standards and to improve the methods of detector handling and data evaluation. 19