In this paper the present status of the Direct Ion Storage Neutron (DIS-N) prototype dosemeter (RADOS) is described. The separation of neutron from photon dose equivalent has been improved by adding tin shieldings. The neutron energy response has been changed by additional plastic covers containing 40% B4C in order to reduce the over-response to thermal neutrons. The responses of the dosemeters were determined for standard photon and neutron fields (monoenergetic neutrons, neutron sources and simulated workplace fields). Irradiations in real workplaces were also performed. The dependence of the neutron response on the angle of incidence was measured for different neutron source

Boschung, M.

Fiechtner, A.

Wernli, C.

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PRESENT STATUS OF THE PERSONAL NEUTRON DOSEMETERBASED ON DIRECT ION STORAGEA. Fiechtner, M. Boschung and C. WernliPaul Scherrer Institut, CH-5232 Villigen PSI, SwitzerlandIn this paper the present status of the Direct Ion Storage Neutron (DIS-N) prototype dosemeter (RADOS) is described. Theseparation of neutron from photon dose equivalent has been improved by adding tin shieldings. The neutron energy responsehas been changed by additional plastic covers containing 40% B4C in order to reduce the over-response to thermal neutrons.The responses of the dosemeters were determined for standard photon and neutron ®elds (monoenergetic neutrons, neutronsources and simulated workplace ®elds). Irradiations in real workplaces were also performed. The dependence of the neutronresponse on the angle of incidence was measured for different neutron sources.DESIGN OF IMPROVED DIS-N DOSEMETERThe personal neutron dosemeter DIS-N (Figure 1),produced by RADOS Technology Oy, is based onionisation chambers with direct ion storage and adouble-chamber system that allows differentialreadings to separate the neutron from the photondose equivalent(1). The photon energy response ofthe two chambers of former DIS-N prototypes wassigni®cantly different in the photon energy regionbelow 100 keV, which could have led to a falsedetermination of the personal neutron dose equiva-lent. The situation has been improved by adding inall dosemeter types a 1-mm-thick tin shieldingaround the chambers to cut off photon energiesbelow 100 keV.Furthermore, former designs tended to an over-response to thermal neutrons, which can be reducedby surrounding the dosemeter with plastic `boron-covers' containing 40% B4C. Four types of dose-meter (Table 1) were built and tested under differentcircumstances. The wall materials of the neutron/photon sensitive chambers are made of A-150 con-taining 1.25% boron nitride (BN) or polyethylene(PE) containing 4% LiNO3. The photon-sensitivechambers were in all cases made of Te¯on (poly-tetra¯ouroethylene) containing 60% graphite. Thesize of a dosemeter is 45 45 15 mm3, and theweight is about 56 g. The readout of the dosemeteris performed with a table-top unit (DBR-1).IRRADIATION CONDITIONSThe dosemeters were mounted for all irradiations onthe ISO water-slab-phantom of size 30 30 15 cm3.The distance to the radiation source was measured fromthe front plane of the phantom and varied between0.5 and 1 m.ENERGY DEPENDENCE OF PHOTONRESPONSEThe energy dependence of the photon response wasdetermined at the calibration laboratory of the PaulScherrer Institut between 24 and 660 keV at normalincidence. Typical results for the neutron/photon-sensitive chamber and the photon-sensitive chamberof the PE (4% LiNO3) dosemeter are shown inFigure 2. Due to the tin shielding, photon energieslower than 100 keV may be neglected. Thus theenergy dependences of the photon response arevery similar, and the separation of neutron fromphoton dose equivalent is improved in mixed neutron/photon ®elds.ENERGY DEPENDENCE OF NEUTRONRESPONSEThe energy dependence of the personal neutron doseequivalent response, shown in Table 2 and Figure 3,Figure 1. DIS-N dosemeter.Corresponding author: annette.®echtner@psi.chRadiation Protection Dosimetry (2004), Vol. 110, Nos 1-4, pp. 213---217doi:10.1093/rpd/nch127Radiation Protection Dosimetry Vol. 110, Nos 1-4 ã Oxford University Press 2004; all rights reserved213was measured for all dosemeter types for monoener-getic neutrons from thermal energies up to 14.8 MeV,for neutron sources and for simulated work-place ®eld spectra. Irradiations with thermalneutrons were performed at the thermal beam atGKSS(2) (Geesthacht, Germany). Irradiations withmonoenergetic neutron energies were performed atthe PTB (Braunschweig, Germany). Two simulatedworkplace ®eld spectra were supplied by IRSN(Cadarache, France). These were a broad, partlythermalised neutron spectrum, `Canel'(3), and ahighly thermalised neutron spectrum, `Sigma'(4).Figure 2. Energy dependence of the Hp(10) photon responses of the neutron/photon-sensitive chamber PE (4% LiNO3)and the photon-sensitive chamber Te¯on/graphite with an integrated 1 mm tin shielding.Table 2. Hp(10) neutron responses of four different dosemeter types to monoenergetic neutrons, neutron sources and simulatedworkplace ®eld spectra, normalised to 241AmBe.Neutron energy(MeV)/neutronsource/®eld spectraPE(4% LiNO3)PE(4% LiNO3)(1 mm B4C)A-150(1.25% BN)A-150(1.25% BN)(2 mm B4C)2.53E ÿ 08 12 3 1.0 0.2 50 15 2.3 0.7Sigma 4.2 0.5 1.0 0.1 21 3 1.1 0.2Canel 2.5 0.5 0.54 0.11 7.9 1.6 0.64 0.130.070 0.45 0.13 0.17 0.06 1.7 0.2 0.14 0.060.144 0.26 0.04 0.14 0.03 0.74 0.09 0.19 0.030.565 0.48 0.06 0.36 0.06 0.63 0.08 0.47 0.08Cf252 (D2O) 0.92 0.09 0.70 0.07 2.2 0.2 0.97 0.10Cf252 (D2O), Cd 0.89 0.09 0.71 0.07 1.6 0.2 0.85 0.09Cf252 0.88 0.09 0.70 0.07 0.90 0.09 0.83 0.08241AmBe 1.0 0.1 1.0 0.1 1.0 0.1 1.0 0.114.8 0.78 0.09 0.78 0.09 0.75 0.09 0.76 0.09CERF concrete ------ 0.42 0.05 0.80 0.10 ------Table 1. Description of DIS-N dosemeter design.Dosemeter type Wall material ofphoton-sensitive chamberWall material of neutron/photon-sensitive chamberAdditional coverPE (4% LiNO3) Te¯on/graphite PE  4% LiNO3 ------PE (4% LiNO3) (1 mm B4C) Te¯on/graphite PE  4% LiNO3 1 mm B4CA-150 (1.25% BN) Te¯on/graphite A-150  1.25% BN ------A-150 (1.25% BN) (2 mm B4C) Te¯on/graphite A-150  1.25% BN 2 mm B4CA. FIECHTNER ET AL.2140.00.20.40.60.81.01.210 100 1000Photon energy (keV)Hp(10) response normalised to 137 CsNeutron/photon sensitive chamber (PE(4%LiNO3))Photon sensitive chamber (Teflon/graphite)Measurements were also performed in a high neu-tron energy ®eld at the CERN reference radiationfacility (CERF)(5,6). The neutron responses to theintermediate neutron energy region (Figure 3) havenot yet been determined, and therefore the dashedlines between 2.5 10ÿ8 and 0.07 MeV are only aguideline. The responses will be determined using adeconvolution method that is described elsewhere(7).The dosemeter types PE (4% LiNO3) and A-150(1.25% BN), without a boron-cover, have a rela-tively ¯at response to neutron energies from 70 keVup to 14.8 MeV and to neutron sources but have asigni®cant over-response to thermal neutrons andto thermalised neutron spectra, especially the A-150(1.25% BN) dosemeter. The boron covers of the dose-meter types PE (4% LiNO3) (1 mm B4C) and A-150(1.25% BN) (2 mm B4C) extenuate this effect. Never-theless, an under-response to neutron energies in thekiloelectronvolt-region (minimum at 144 keV) andalso to the intermediate neutron energy region wasobserved with these dosemeter types.ANGLE DEPENDENCE OF THE NEUTRONRESPONSEThe angle dependence of the neutron response wasmeasured at the Paul Scherrer Institut at angles ofincidence of ÿ75, ÿ45, 0, 45 and 75(vertical axis) for the 241Am---Be and 252Cf (D2O,Cd) neutron sources. The results are presented inFigures 4 and 5. Below 45, the relative neutronresponse changes less than 40%. At an angle ofFigure 3. Energy dependence of the Hp(10) neutron responses of four different dosemeter types to monoenergeticneutrons, normalised to 241AmBe.Figure 4. Hp(10) neutron responses to241AmBe as a function of the angle of incidence to the vertical axis of four differentdosemeter types.PRESENT STATUS OF THE DIRECT ION STORAGE PROTOTYPE DOSEMETER2150.11101001.0E-081.0E-071.0E-061.0E-051.0E-041.0E-031.0E-021.0E-011.0E+001.0E+011.0E+02Neutron energy (MeV)Hp(10) neutron response,normalised to241 AmBePE(4% LiNO3) A-150(1.25% BN)(2 mm B4C)PE(4% LiNO3)(1 mm B4C) A-150(1.25% BN)0.00.20.40.60.81.01.21.41.61.82.0-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90Angle [degree]Neutron response to241 AmBe normalized to 0°PE(4% LiNO3)PE(4% LiNO3)(1 mm B4C)A-150(1.25% BN)A-150(1.25% BN)(2 mm B4C)75 the response changes asymmetrically between0.2 and 2.5, depending on the spectrum/neutronenergy. For spectra with a large number of fastneutrons, like 241AmBe, the angle dependence ofthe neutron response is much less pronounced thanfor spectra with more thermalised neutrons, like252Cf (D2O, Cd). The asymmetry of the angle depen-dence is caused by the geometrical con®guration ofthe neutron/photon-sensitive chamber and thephoton-sensitive chamber inside the dosemeter,where one chamber shields the other chamber foran extreme angle like 75.FIELD EXPERIMENTSWithin the project EVIDOS (`Evaluation of indivi-dual dosimetry in mixed neutron and photon radia-tion ®elds'), which is funded by the EC within the5th framework, measurement campaigns at realworkplaces were and will be performed. The ®rstmeasurement campaign was carried out at a boilingwater reactor and at a storage cask with used fuelelements at the nuclear power plant Krummel inGermany. The determination of reference valuesfor the personal dose equivalent is described else-where(8). The neutron energy spectra have in generala distribution in the lower kiloelectronvolts andintermediate energy region. For the irradiations,only dosemeters with boron covers were used [PE(4% LiNO3) (1 mm B4C) and A-150 (1.25% BN)(2 mm B4C)]. The measured personal neutron doseequivalent rates are compared with preliminary doserates of an area monitor (LB6411) (Table 3). For themeasurement point `SAR, boiling reactor' the ratioof the personal photon dose equivalent to the per-sonal neutron dose equivalent is about 3, which is atthe present development status of the dosemeter toohigh for the measurement of a neutron dose. At themeasurement point `Top, boiling reactor', the irrad-iation time was too short, and measured doses werepartly below the detection limit. The results indicatethat these dosemeter types are not suitable for neu-tron spectra with a high proportion of intermediateneutron energies because the response to these ener-gies is too low and correspondingly the detectionlimit would be too high.Figure 5. Hp(10) neutron responses to252Cf (D2O, Cd) as a function of the angle of incidence to the vertical axis of fourdifferent dosemeter types.Table 3. Preliminary results of the ®rst measurement campaign in real workplaces at the nuclear power plant KruÈmmel inGermany.Measurement point PE (4% LiNO3)(1 mm B4C) Hp(10)n (mSv hÿ1)A-150 (1.25% BN)(2 mm B4C) Hp(10)n (mSv hÿ1)LB6411H(10)n (mSvÿ1 hÿ1)Cask midline 12 10 63 20 140 30Cask side 10 5 13 6 50 10SAR, boiling reactor   60 12Top, boiling reactor 15 15  40 8Below detection limit.A. FIECHTNER ET AL.2160.00.51.01.52.02.53.0-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90Angle [degree]Neutron response to252 Cf(D2O, Cd) normalised to 0°PE(4% LiNO3)PE(4% LiNO3)(1 mm B4C)A-150(1.25% BN)A-150(1.25% BN)(2 mm B4C)CONCLUSIONSThe experiments did not indicate if the use of boroncovers (surrounding the dosemeter) is recom-mendable. A second measurement campaign withinEVIDOS was carried out at the research reactorVENUS at CEN/SCK, and the fuel processingplant of Belgonucleaire in Belgium, where mostneutrons are high-energy neutrons or are only partlythermalised. The preliminary results showed that thePE (4% LiNO3) dosemeter without a boron covercould be a useful compromise for broad neutronenergy spectra as long as it is not used in highlythermalised neutron spectra. Which type of dose-meter is optimal depends, therefore, on the ®eld ofapplication.The construction of a dosemeter that is only cov-ered on the front side with a boron cover (similar tothe principle of an albedo dosemeter) should beconsidered, to maintain a high enough sensitivityto intermediate neutrons along with a reasonableresponse to thermal neutrons.Apart from the discussion regarding which typeof dosemeter is useful for which type of spectrum,all dosemeter types still have the drawback thatthey are only usable in mixed photon/neutron®elds as long as the personal photon dose equiva-lent is not greater than twice the personal neutrondose equivalent. Otherwise the detection limitof the personal neutron dose equivalent exceeds100 mSv.ACKNOWLEDGEMENTSSpecial thanks to Marlies Luszik-Bhadra for fruitfuldiscussions and to the staff of the GKSS facility fortheir assistance during the measurements. This workwas partly performed within the European Commis-sion RTD Programme, Nuclear Energy, EuratomFramework Programme V, 1998---2002, ContractNo. FIKR-CT-2001-00175, and was funded partlyby the Federal Of®ce of Education and Science(BBW contract No. 01.0024).REFERENCES1. Fiechtner, A., Wernli, C. and Kahilainen, J. A proto-type personal neutron dosemeter based on an ion cham-ber and direct ion storage. Radiat. Prot. Dosim.96(1---3), 269---272 (2001).2. BoÈttger, R., Dietz, E., Friedrich, H., Lagner, F.,Luszik-Bhadra, M. and Sosaat, W. Thermal neutroncalibration ®eld for radiation protection. GKSS report2002/4 (2002).3. Lacoste, V., Gressier, V. and Bolognese-Milsztajn, T.Monte-Carlo simulation of the neutron and dosimetriccharacteristics at the IRSN Canel/T400 facility. Radiat.Prot. Dosim. 110(1---4), 123---127 (2004).4. Muller, H., Gressier, V., Lacoste, V., Lebreton, L. andPochat, J.-L. Characterization of the thermal neutron®eld produced by the IRSN sigma facility. Radiat. Prot.Dosim. 110(1---4), 136---140 (2004).5. Schuhmacher, H., Brede, H. J., Dangendorf, V. andKuhfuss, M. Quasi-monoenergetic neutron beams withenergies from 25 to 70 MeV. Nucl. Instrum. Meth.Phys. Res. A 421, 284---295 (1999).6. Birattari, C., Ferrari, A., HoÈfert, M., Otto, T., Rancati, T.and Silari, M. Recent Results at the CERN-ECHigh Energy Reference Field Facility. Third Specia-lists' Meeting on Shielding Aspects of Accelerators,Targets and Irradiation Facilities, Sendai, Japan,12---13 May 1997 (Paris: OECD) pp. 219---234 (1998).7. Reginatto, M. et al. Dose equivalent response of neutrondosemeters determined using unfolding methods.Presented at the Eleventh International Congress ofthe International Radiation Protection Association,Madrid, Spain, 23---28 May 2004.8. Luszik-Bhadra, M. et al. EVIDOS: evaluation ofindividual monitoring in mixed neutron/photon ®eldsat workplaces in the nuclear fuel cycle. Presented at theFourth EC/IAEA/ISOE Workshop on OccupationalExposure Management at NPP's, Lyon, France,24---26 March (2004).PRESENT STATUS OF THE DIRECT ION STORAGE PROTOTYPE DOSEMETER217

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