Neutron survey instruments have been exposed at all the measurement locations used in the EVIDOS project. These results have an important impact in the interpretation of the results from the project, since operationally the survey instrument will be used for an initial assessment of and routine monitoring of the ambient dose equivalent dose rate. Additionally, since the response of these instruments is in some cases very well characterised, their systematic deviations from the reference quantities provide an important verification of the determination of those quantitie

Bolognese-Milsztajn, T.

Boschung, M.

Coeck, M.

Curzio, G.

d'Errico, F.

Fiechtner, A.

Hager, L. G.

Hussien, M.

Kyllönen, J.-E

Lacoste, V.

Lindborg, L.

Luszik-Bhadra, M.

Molinos, C.

Reginatto, M.

Schuhmacher, H.

Tanner, R. J.

Vanhavere, F.

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NEUTRON AREA SURVEY INSTRUMENT MEASUREMENTSIN THE EVIDOS PROJECTR. J. Tanner1,, T. Bolognese-Milsztajn2, M. Boschung3, M. Coeck4, G. Curzio5, F. d’Errico5,A. Fiechtner3, L. G. Hager1, M. Hussien1, J.-E. Kyllo¨nen6, V. Lacoste2, L. Lindborg6,M. Luszik-Bhadra7, C. Molinos8, M. Reginatto7, H. Schuhmacher7 and F. Vanhavere41Radiation Protection Division, Health Protection Agency, Chilton, Didcot OX11 0RQ, UK2Institut de Radioprotection et de Suˆrete´ Nucle´aire, F-92265 Fontenay-aux-Roses, France3Paul Scherrer Institut, CH-5232 Villigen, Switzerland4Studiecentrum voor Kernenergie—Centre d’e´tude de l’e´nergie nucle´aire, B-2400 Mol, Belgium5Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione, I-56126 Pisa, Italy6Swedish Radiation Protection Authority, SE-171—16 Stockholm, Sweden7Physikalisch-Technische Bundesanstalt, D-38116 Braunschweig, Germany8Formerly, NRPB, Chilton, Didcot, Oxon, OX1 4TR, UKNeutron survey instruments have been exposed at all the measurement locations used in the EVIDOS project. These resultshave an important impact in the interpretation of the results from the project, since operationally the survey instrument willbe used for an initial assessment of and routine monitoring of the ambient dose equivalent dose rate. Additionally, sincethe response of these instruments is in some cases very well characterised, their systematic deviations from the referencequantities provide an important verification of the determination of those quantities.INTRODUCTIONNeutron survey instruments are important toolsfor controlling doses to individuals in the workplace.They are used routinely to determine the dose rates inareas where persons may be occupationally exposed,to delineate controlled areas, to prospectively estab-lish systems of work, and to monitor dose rate levels.Most of the instruments in general use are of themoderator-type. Such instruments have less thanperfect energy dependence of ambient dose equiva-lent response characteristics, over-responding in theenergy region between 1 eV and 10 keV.Measurements were made at each of the work-places studied in the EVIDOS project(1) using arange of commercially available instruments: Bert-hold LB6411, Harwell Instruments N91, Studsvik2202D and WENDI-II. These results are interpretedin terms of the known energy dependence of responseof the instruments and the measured fluence-energydistributions of the fields. The ability of these instru-ments to provide accurate assessments of H(10) andconservative estimates of Hp(10) is hence qualified.Whilst the main focus of the EVIDOS project hasbeen on personal dosemeters, the ‘front line’ forradiation protection in the workplace is the initialsurvey that is made to assess potential occupationaldoses. The accuracy of this survey is of primaryimportance: Overestimates of the dose rates may affect opera-tional efficiency by restricting work practices. Underestimates of dose rates may lead to under-estimates of the risks to workers from working inspecific environments.Consequently, the accuracy of radiation surveysis highly significant. Within the EVIDOS project, theambient dose equivalent readings of this selection ofsurvey instruments will be compared to the personaldose equivalent rate at the measurement positions.Additional measurements were made with TEPCbased systems, which can also be used to determinethe ambient dose equivalent rate. One such system,and its results in the EVIDOS project, is discussedelsewhere in these proceedings(2).RESPONSE OF SURVEY INSTRUMENTSMost of the neutron survey instruments in this pro-ject use 1/v proportional counters, based on eitherthe 3He(n, p) or 10B(n, a) reactions. Consequently,they rely on moderation of the field by a large hydro-genous mass to slow down the neutrons and enhancethe detection efficiency for fast neutrons. The energydependence of the response is then modified by theuse of a perforated thermal neutron absorbing layer,cadmium or boron-loaded rubber, which ensuresthat the H(10) response of the instrument is flatterthan would otherwise be the case. In the case ofthe WENDI-II this layer is made from unperforatedtungsten.In practice, the ambient dose equivalent res-ponse of neutron survey instruments varies signi-ficantly with energy (Figure 1). In particular, theresponse peaks in the 1–10 keV region, just belowCorresponding author: rick.tanner@hpa-rp.org.ukRadiation Protection Dosimetry (2007), Vol. 125, No. 1–4, pp. 300–303 doi:10.1093/rpd/ncm160Published by Oxford University Press 2007the energy range where the fluence to dose equivalentconversion coefficient begins to rise steeply. Gene-rally, workplace fields are determined as having asmall component of dose equivalent in the energyrange where these moderator-type instrumentsoverestimate significantly.The EVIDOS contract offers a unique oppor-tunity to study the effect of the angle dependenceof response of these instruments, since the directiondependence of the fields is being determined at themeasurement locations(5). Unfortunately, there arenot detailed energy and angle dependence of res-ponse data available for the LB6411 or WENDI-II,which will limit this aspect of the analysis. There are,however, detailed data available for the Studsvik2202D and for a different model of the Leake design,the 0949(2): these data show that the angle depen-dence is less important than the energy dependenceof response, except when the instrument is exposedthrough its electronics. In real workplaces, withsignificant scatter, this will always account for acomponent of the field, but when used wisely, itshould not constitute a very significant fraction ofthe field.RESPONSE IN EVIDOS FIELDSThe H(10) rate measured using each of the instru-ments, divided by that calculated directly from theBonner sphere determinations of the fluence-energydistribution(6,7), gives the bias in the reading of theinstrument. A theoretical value for the bias can alsobe obtained by folding the instrument response withthe energy distribution and dividing this value by thereference H(10).The bias in the H(10) reading is of primary inter-est. However, the ratio of the measured H(10) to thereference value for Hp(10) is also of interest: if themeasurement of H(10) is a significant underesti-mate, then the instrument could underestimateHp(10) also; if H(10) is overestimated when it issignificantly larger than Hp(10), then the readingcan be excessively conservative.The relationship between H(10) and Hp(10, y) iscomplex. For y¼ 0 the two quantities are very simi-lar, but Hp(10) is much smaller than H(10) for largeangles of incidence for all neutron energies encoun-tered in the EVIDOS project. The two quantities aremost different in highly scattered fields.Three parameters are plotted in Figure 2: Ratio of the measured ambient dose equivalentrate, H(10)M, to the reference value(6,7),H(10)R. Ratio of the calculated response of the instru-ment, H(10)C, to H(10)R. Differences betweenthis ratio and the measured ratio will derive fromdeficiencies in the response data, the angle depen-dence of response of the instrument or problemswith the determination of H(10)R. Ratio of the reference personal dose equiva-lent(5), Hp(10)R, to H(10)R. The value of thisratio should be lower than H(10)M/H(10)R.The response calculated for each instrumentused the same calibration as was applied for themeasurements: The Studsvik 2202D was calibrated at HPAusing a bare 241Am-Be source: 1242 mSv1. The N91 used the manufacturer’s recommendedvalue of 864 mSv1.Figure 1. H(10) response of the 2202D(2), LB6411(3), WENDI-II(4) and Leake(2) designs, from their reference direction.NEUTRON SURVEY MEASUREMENTS IN THE EVIDOS PROJECT301 H(10) readings were taken directly from theLB6411 display. The instrument is calibratedin terms of a bare 252Cf source, for which themanufacturer specifies a response of 2830 mSv1. A calibration of 2640 mSv1 was used for theWENDI-II based on a calibration using a bare241Am-Be source at PTB.Figure 2 and Table 1 show the results from theEVIDOS project. A few data points are missing: theratio of Hp(10)R/H(10)R for the VENUS ControlRoom, for which the dose rate was too low for areliable Hp(10) determination; WENDI-II mea-surement in the CANEL field; WENDI-II and N91measurements in the SIGMA field.Generally, the measured results show good agree-ment with the reference values for H(10). Comparedto these reference values, the mean biases for theinstruments are relatively small, ranging from 9%for the LB6411 to þ15% for the WENDI-II. Thepotential for positive biases is greater, with þ76%Figure 2. Results from the Studsvik 2202D, N91, LB6411 and WENDI-II in the EVIDOS fields: ~¼ the ratio of themeasured H(10) to the reference value; ~¼ the ratio of the calculated response to the measured response; &¼ the ratioof the reference value for Hp(10) to that for H(10). Responses have been calculated for instrument/field combinationsirrespective of whether a measurement was made. There is no value potted for the ratio of the dose equivalent quantitiesfor the VENUS Control Room because there is no reference value for Hp(10) for that location.R. J. TANNER ET AL.302for the WENDI-II in the KKK SAR field being themaximum value. Negative biases are never greaterthan 35% for the LB6411 in the BN-3 field.Negative biases are of concern where they can leadto an underestimate of Hp(10). The field that comesclosest to this situation is the BN-2b field, a relativelyunidirectional, moderated field. Here the H(10)measurement and Hp(10) value effectively coincidewithin experimental uncertainties.The predicted and measured responses show gen-erally good agreement. The full analysis of these datawill require consideration of the direction depen-dence of the field, although this aspect of the analysiscan only be properly undertaken for the 2202D,which has full energy and angle dependence ofresponse data available.CONCLUSIONSThe accuracy of the survey instrument readings inthe EVIDOS project will be significant in terms ofthe interpretation of the final results of the project.Significant biases in survey instrument readings canhave implications for either workplace practice orthe assessment of personal and effective dose.In general, the results show small underestimatesof H(10), although the Kru¨mmel BWR fields are anexception: since those fields have high H(10)/Hp(10)ratios, the readings of ambient dose equivalentprovide substantial overestimates of Hp(10).Since one of the instruments has detailed energyand angle dependence of response data available, itsreadings can also provide important checks on theF(E) and F(E, O) distributions determined in thiswork. This aspect of the work is incomplete.REFERENCES1. d’Errico, F., Bartlett, D., Bolognese-Milsztajn, T.,Boschung, M., Coeck, M., Curzio, G., Fiechtner, A.,Kyllo¨nen, J.-E., Lacoste, V., Lindborg, L. et al. Evalu-ation of Individual Dosimetry in mixed neutron and pho-ton radiation fields (EVIDOS). Part I: Scope andmethods of the project. These proceedings.2. Kyllo¨nen, J. and Lindborg, L. Photon and neutrondose discrimination using low pressure proportionalcounters with graphite and A150 walls. Theseproceedings.3. Tanner, R. J., Molinos, C., Roberts, N. J.,Bartlett, D. T., Hager, L. G., Jones, L. N.,Taylor, G. C. and Thomas, D. J. Practical implicationsof neutron survey instrument performance. To bepublished as an HPA Report (2005).4. Burgkhardt, B., Fieg, G., Klett, A., Plewnia, A. andSiebert B. R. L. The neutron fluence and H(10)response of the new LB6411 remcounter. Radiat ProtDosim, 70, 361–364 (1997).5. Olsher R. H., Hsiao-Hua H., Beverding A., Kleck J. H.,Casson W. H., Vasilik D. G. and Devine R. T. WENDI:an improved neutron rem meter. Health Phys, 79,170–181 (2000).6. Luszik-Bhadra, M., Schuhmacher, H., Bolognese-Milsztajn, T., Boschung, M., Coeck, M., Curzio, G.,d’Errico, F., Fiechtner, A., Kyllo¨nen, J.-E., Lacoste, V.et al. Directional distributions of neutrons and referencevalues of personal dose equivalent in workplace fields.These proceedings (2005).7. Lacoste, V. and Gressier, V. Monte carlo simulation ofthe operational quantities at the realistic mixed neutron-photon radiation fields CANEL and SIGMA. Theseproceedings (2005).8. Lacoste, V. Asselineau, B. Muller, H. and Reginatto M.Bonner sphere neutron spectrometry at nuclear work-places in the framework of the EVIDOS Project. Theseproceedings (2005).Table 1. Bias, the ratio of the measured H(10) rate to the reference value for the four instruments.Bias 2202D N91 LB6411 WENDI-IIMaximum þve 1.20 (0.13) (KKK Top) 1.47 (0.16) (KKK Top) 1.28 (0.13) (KKK SAR) 1.76 (0.14) (KKK SAR)Mean 0.96 (0.03) 1.05 (0.06) 0.91 (0.04) 1.15Maximum –ve 0.79 (0.08) (BN-2b) 0.76 (0.08) (BN-2b) 0.65 (0.08) (BN-3) 0.78 (0.07) (BN-2b)The mean bias is the average for the 16 fields, with the quoted uncertainty deriving from the distribution of values. Themaximum positive and negative biases have uncertainties quoted that include the calibration uncertainty and countingstatisticsNEUTRON SURVEY MEASUREMENTS IN THE EVIDOS PROJECT303

Neutron area survey instrument measurements in the EVIDOS project

Neutron area survey instrument measurements in the EVIDOS project

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