Evaluation of spectrum measurement devices for operational use

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  • Nuclear Instruments and Methods in Physics Research A 476 (2002) 416422

    Evaluation of spectrum measurement devices foroperational use

    Robert T. Devine*, Leonard L. Romero, Devin W. Gray, David T. Seagraves,Richard H. Olsher, Je P. Johnson

    Los Alamos National Laboratory, MS G761 ESH-4, P.O. Box 1663, Los Alamos, NM 87545, USA

    Abstract

    Several neutron spectrometers manufactured by Bubble Technology Industries (BTI) were tested and evaluated in a

    variety of neutron elds. Findings and conclusions are presented for the following BTI instruments: a modication ofthe Rotational Spectrometer (ROSPEC) that includes a thermal and epithermal capability, the Simple ScintillationSpectrometer that is used in conjunction with the ROSPEC to extend its high-energy range, and the MICROSPECN-Probe which is capable of providing a crude spectrum over the energy range from thermal to 18MeV. The main

    objective of these measurements was to determine the accuracy of both the energy spectrum and dose equivalentinformation generated by these devices. In addition, the dose response of the Wide-Energy Neutron DetectionInstrument (WENDI-II) was measured in all neutron elds relative to a bare 252Cf calibration. The performance of the

    WENDI-II rem meter was compared to the dose information generated by the neutron spectrometers. The instrumentswere irradiated to bare 252Cf and 241AmBe sources, and in a series of moderated 252Cf elds using a standard D2Osphere and a set of polyethylene spheres. The measured spectra were benchmarked with a set of detailed Monte Carlo

    calculations with the same energy bin structure as that of the instruments under test. These calculations allowed anabsolute comparison to be made with the measurements on a bin by bin basis. The simulations included the eects ofroom return and source anisotropy. r 2002 Elsevier Science B.V. All rights reserved.

    PACS: 87.53.Qc

    Keywords: Neutron dosimetry measurements

    1. Introduction

    Four devices were investigated:Rotational Spectrometer [1] (ROSPEC). This

    consists of four spherical gas counters arrayed on arotating table with a range 504.5MeV. Theparticular version used is enhanced with two

    additional detectors to measure the range fromthermal to 50 keV (bare 6LiI scintillator for thermalneutrons, and another 6LiI scintillator shielded by10B for detection of epithermal neutrons).Simple Scintillation Spectrometer [2] (SSS).

    This is an array of small plastic scintillatorscoupled to a photomultiplier tube with a rangeof 415MeV. The ROSPEC and SSS are used incombination.N-Probe [3]. This device uses a boron shielded

    6LiI for the range thermal to 0.8MeV and a

    *Corresponding author. Tel.: +1-505-667-4254; fax: +1-

    505-665-6071.

    E-mail address: devine r@lanl.gov (R.T. Devine).

    0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

    PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 4 8 1 - 4

  • NE213 scintillator for the range 0.8MeV to about18MeV.WENDI II [4]. This a neutron rem meter

    incorporating a shell of tungsten powder insidea polyethylene moderator, surrounding a 3Hedetector.

    2. Experimental and analytical methods

    LANL Low-Scatter Neutron Facility was usedfor the exposures. This facility consists of a sheetmetal structure (8m 5.3m 6m) housing a plat-form made of an aluminum lattice. The primaryscattering boundary is located 3m below thealuminum platform. Detectors were positioned ata height of 60 cm above the platform. Monte Carlosimulations were done with MCNP [5]. The uencewas calculated using the same energy bin structureas that of the spectrometers under test for all of thereference elds. The appearance of the ROSPEC/

    SSS and the N-Probe spectra will dier since thebins used in the calculation correspond to theoutput bins of the instruments which are not thesame. The calculations were based on the actualexposure conditions in that the total ux for eachindividual exposure was input to the MCNPcalculation. The eects of room return and sourceanisotropy were included in the simulations. Allcalculations were carried out to 5% uncertainty.

    3. Results and discussion

    3.1. ROSPEC/SSS

    For bare 252Cf, D2O moderated252Cf and

    241AmBe the spectra shown in Fig. 1 with goodagreement with calculation in the 50 keV13.8MeV region. They are referred to in Figs. 1and 3 and the Tables as BCF, MCF, and AmBe,respectively. The uence results in three energy

    Fig. 1. ROSPEC/SSS measured and calculated spectra: BCF, MCF and AmBe (ROSPEC/SSS, solid line, MCNP dotted line, see text

    for abbreviations).

    R.T. Devine et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 416422 417

  • bins (Slow (ThermalF53.5 keV, thermal andepithermal 6LiIdetectors), Intermediate(53.5 keV4.53MeV, ROSPEC) and Fast (4.5313,2MeV, SSS), is given in Table 1. For bare252Cf, the total uence is over reported by 25%,due to the over response in the Slow bin. However,

    the dose equivalent contribution from the Slow binis 1% and H 10 agrees with the calculated datawithin 6%. There is a factor of two overestimate ofthe total uence for the D20 moderated spectrumagain mainly due to over response in the Slow binwhich in this case contributes 17% of the dose.

    Table 1

    ROSPEC/SSS uence results (see text for denitions and abbreviations)

    Reference eld MCNP calculated uence (n/cm2) ROSPEC/SSS measured uence (n/cm2)

    Slow Intermediate Fast Total Slow Intermediate Fast Total

    BCF 8.21e5 5.73e7 5.41e6 6.35e7 1.93e7 5.40e7 5.75e6 7.91e7

    MCF 3.62e7 1.74e7 1.50e6 5.51e7 8.82e7 1.74e7 1.68e6 1.07e8

    Ambe 2.11e5 6.59e6 3.16e6 9.96e6 3.28e6 6.03e6 3.94e6 1.33e7

    p2 4.18e6 4.97e7 4.65e6 5.86e7 2.71e7 4.74e7 5.03e6 7.95e7

    p3 1.04e7 4.39e7 4.01e6 5.83e7 4.04e7 3.94e7 4.37e6 8.41e7

    p5 2.00e7 3.09e7 2.99e6 5.39e7 5.58e7 2.87e7 3.06e6 8.75e7

    p8 1.86e7 1.65e7 1.91e6 3.70e7 3.07e7 1.43e7 2.34e6 4.74e7

    p10 1.35e7 1.05e7 1.42e6 2.55e7 2.74e7 9.08e6 1.33e6 3.78e7

    p12 8.96e6 6.73e6 1.05e6 1.67e7 1.33e7 6.20e6 1.72e6 2.12e7

    Fig. 2. ROSPEC/SSS measured and calculated spectra for polyethyene spheres (ROSPEC/SSS, solid line, MCNP dotted line, see text

    for abbreviations).

    R.T. Devine et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 416422418

  • The AmBe total uence is overestimated by 34%by a large Slow bin contribution but in this casethe Slow bin contribution to the dose is again 1%.If the ROSPEC alone is used for detection ofAmBe the dose is under reported by 27% whichwas to be expected.Spectra of 252Cf moderated by polyethylene

    spheres of 2, 3, 5, 8, 10, and 12 in. diameter areshown in Fig. 2. These are referred to in Figs. 2and 4 and the Tables as p2; p3; p5; p8; p10; andp12; respectively. The successively larger modera-tion yields spectra that dier from bare califor-nium by larger contributions to the thermal andepithermal regions. There is good agreement withthe calculated spectrum over the energy rangefrom 0.1 to 13MeV with unfolding artifactsbetween 50 and 100 keV and at 4.5MeV causedby the joining of the spectra calculated from thevarious components of the ROSPEC/SSS combi-nation. An anomaly in the spectra for the 10- and12-in. polyethylene is zero uence 1 eV to 10 keV.The manufacturer attributes this to an algorithmthat reports zero when the uence falls below agiven value. Estimated dose equivalents are givenin Table 2. The results for these elds are within8% [H 10] of the calculated dose equivalentand the contribution from the Slow bin is less than6%.Several measurements (with identical count

    times) of a D2O moderated252Cf spectrum were

    performed over a period of two months. Thesemeasurements indicate minimal drift and repro-ducible performance over all energy bins. The SP6

    detector covers the upper energy range of thestandard ROSPEC. Over the past two years, thisdetector has had a history of becoming noisy forno apparent reason. The problem is intermittentand may clear up several days after the system isturned o. The problem has not been resolvedeven after a trip to the factory for repair. Artifactscreated by the algorithm in the juncture of thelower end of the spectra obtained from the SSSand the high end of ROSPEC are more pro-nounced and the total reported uence is reducedsignicantly. It is clearly essential to conrm thatall of the detector channels are noise free prior toand immediately after each data collection.

    3.2. MICROSPEC/N-Probe

    Bare 252Cf, D2O moderated252Cf, and 241AmBe

    spectra are shown in Fig. 3 and spectra forpolyethylene spheres shown in Fig. 4. Fluenceresults in three energy bins (Slow (Ther-malF50 keV), Intermediate (50keV5MeV andFast (5MeV16MeV), are given in Table 3. Forbare 252Cf, the total uence is over reported by 6%,due to the over response in the Slow bin. However,the dose equivalent contribution from the Slow binis 1% and H 10 agrees with the calculated datawithin 6%. There is factor of two overestimate ofthe total uence for the D20 moderated spectrumagain mainly due to over response in the Slow binwhich in this case contributes 18% of the dose. TheAmBe total uence is overestimated by 11% alsoby overestimate in the Slow bin but in this case the

    Table 2

    Results. Normalized to the MCNP calculation for each spectrum or bare Cf (see text for denitions and abbreviations)

    Reference eld ROSPEC H 10: ROSPEC/SSS H 10 N-probe H 10 WENDI H 10MCNP=1 MCNP=1 MCNP=1 Cf=1.00

    BCF 0.96 1.06 1.06 FMCF 1.18 1.28 1.34 1.29

    Ambe 0.73 1.24 1.15 0.98

    p2 0.98 1.08 1.05 1.00

    p3 0.94 1.04 1.01 1.01

    p5 0.98 1.08 0.99 1.05

    p8 0.91 1.05 0.95 1.01

    p10 0.84 1.02 1.07 1.00

    p12 0.98 1.02 1.01 0.95

    R.T. Devine et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 416422 419

  • Slow bin contribution to the dose is 2%. With theexception of D2O moderated californium the overresponse in the Slow bin results in an overestimateof the total uence that ranges from 26% to+13%. Dose reported is shown in Table 2 with avariation in H 10 of 5% to +34%

    3.3. WENDI-II neutron rem meter

    Normalized dose response summarized in Table2 with H 10 variation of 5% to +29%. Thetraditional energy response (response per unit doseequivalent) of all moderated rem meters is far fromideal in the energy range from 10 eV to 50 keV,with an over response as large as a factor of nine atan energy of 5 keV. This means that in unusualoperational cases where a signicant fraction ofthe dose is contributed by sub-50 keV neutrons,rem meters may over respond by a factor of two ormore.

    4. Conclusions

    ROSPEC (without the SSS) accurately measuresneutron spectra from 0.1 to 4.5MeV. Its resolutionis exceptional and exceeds that of the traditionalBonner sphere spectrometer. Thermal and epither-mal energy bins are signicantly over estimated.Dose contribution in this region for most work-place spectra [6] is no more than 10%; the practicaleect of this over response is minor in terms of thereported dose.SSS provides good resolution and accuracy over

    its range of operation (415MeV). SSS greatlyimproves the dosimetric accuracy of the standardROSPEC for measurements in hard spectra.N-Probe gives a much coarser spectrum. The

    transition region between the fast and thermalscintillators is in many cases inaccurate with asignicant bias in the 10 keV to 0.8MeV energyrange. However, its portability and ease of usemake it attractive as a smart neutron rem meter

    Fig. 3. N-Probe measured and calculated spectra: BCF, MCF and AmBe (N-probe, solid line, MCNP dotted line, see text for

    abbreviations).

    R.T. Devine et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 416422420

  • for eld dose determinations. Its dosimetricperformance was similar to that of the LANL/ROSPEC and the WENDI-II rem meter.In many workplace environments, where the

    dose is dominated by the spectral componentsabove 0.1MeV, the WENDI-II Rem Meter canprovide accurate dosimetric information, compar-

    able to that available from much more expensivespectrometers.The results of these evaluations should allow an

    informed choice to be made of which instrumentsto use in particular situations based on the spectraldetail required by the operational conditions andthe diculties of handling the equipment.

    Fig. 4. N-Probe measured and calcuated spectra for polyethyene spheres (N-Probe, solid line, MCNP dotted line, see text for

    abbreviations).

    Table 3

    N-probe uence results (see text for denitions and abbreviations)

    Reference eld MCNP Calculated uence (n/cm2) N-Probe measured uence (n/cm2)

    Slow Intermediate Fast Total Slow Intermediate Fast Total

    BCF 2.11e6 8.58e6 6.33e5 1.13e7 3.04e6 8.50e6 5.13e5 1.20e7

    MCF 2.01e7 4.32e6 4.85e5 2.49e7 4.47e7 4.52e6 4.40e5 4.96e7

    AmBe 1.55e6 5.78e6 2.63e6 9.96e6 2.45e6 5.84e6 2.77e6 1.11e7

    p2 2.96e6 7.14e6 5.47e6 1.06e7 0.00e0 7.40e6 4.46e5 7.84e6

    p3 4.20e6 6.03e6 4.75e5 1.07e7 5.40e6 5.97e6 3.80e5 1.18e7

    p5 5.50e6 4.11e6 3.55e5 9.97e6 6.92e6 4.10e6 2.81e5 1.13e7

    p8 4.45e6 2.22e6 2.31e5 6.90e6 4.04e6 2.13e6 1.74e5 6.34e6

    p10 1.58e7 7.98e6 1.06e6 2.48e7 1.02e7 7.51e6 1.04e6 1.88e7

    p12 1.04e7 5.19e6 7.95e5 1.64e7 6.85e6 4.77e6 8.10e7 1.24e7

    R.T. Devine et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 416422 421

  • References

    [1] H. Ing, T. Cliord, T. McLean, W. Webb, T. Cousins,

    J. Dhermain, Radiat. Prot. Dosim. 70 (1997) 273.

    [2] Bubble Technology Industries, Inc, Simple Scintillation

    Spectrometer BTI-SSSt for Neutron Spectroscopy to

    15MeV, Revision 4 (July 13, 1999).

    [3] H. Ing, Personal Communication from Harry Ing to Robert

    Devine (October 18, 1999).

    [4] R.H. Olsher, H.H. Hsu, A. Beverding, J.H. Kleck, W.H.

    Casson, D.G. Vasilik, R.T. Devine, Health Physics 79

    (2000) 170.

    [5] Transport Methods Group, Los Alamos National Labora-

    tory, MCNPFA General Monte Carlo N-ParticleTransport Code Version 4B (March 20, 1997).

    [6] W.H. Harvey, F. Hajnal, Radiat. Prot. Dosim. 50 (1993) 13.

    R.T. Devine et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 416422422

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