Neutron and gamma fields at neutron spallation sources for single ...

Proc. RADECS 2013 PB-10L 1/4

Neutron and gamma fields at neutron spallation sources for single-event-effects testing S. P. Platt, Q. Hong, S. J. Mein, L. H. Zhang School of Computing, Engineering and Physical Sciences University of Central Lancashire Preston, Lancashire, PR1 2HE England

Abstract—Monte-Carlo simulations of spallation fields are used to determine neutron and gamma spectra in beams used for single-event-effects testing. Spectra at LANSCE (ICE House) and TSL (ANITA) are calculated using Geant4 with Bertini and binary intranuclear cascade (INC) models. Results with the binary INC model give a good representation of neutron spectra obtained from measurements and independent calculations in each case, and predict gamma dose rates consistent with measured data where these are available. The gamma field in each beam is dominated by a continuum between about 100 keV and about 10 MeV. Integral gamma yields are approximately 0.26 γ p−1 sr−1 and 0.046 γ p−1 sr−1 at the ICE House and ANITA, respectively.

I. I NTRODUCTION Spallation neutron sources are widely used for accelerated testing for single-event effects (SEE) [1], [2], [3], [4], [5]. We are interested in using photodiodes to measure local neutron fluence during such tests [6], [7], [8]. One of the issues in the use of photodiodes is their potential gamma sensitivity. We could not find information in the literature to describe gamma fields in spallation sources used for SEE testing, and have engaged on a study using Monte Carlo techniques to investigate their significance. This work is part of a larger project to study effects in photodiodes for local neutron fluence measurement. II. M ODELLING Models of neutron sources at the Los Alamos Neutron Science Center (LANSCE) Weapons Neutron Research (WNR) Target 4 [9] and the ANITA beam at the University of Uppsala The Svedberg Laboratory (TSL) [5] have been implemented and simulated using Geant 4 [10], [11]. In each simulation a proton beam is axially incident on a cylindrical tungsten target. Model parameters are listed in Table I. Simulations were repeated using QGSP BERT HP and QGSP BIC HP physics lists, employing Bertini and binary intranuclear cascade models, respectively (and referred to as “Bertini” and “binary”, below). The resulting neutron and gamma fields are observed at the specified ranges in air at forward angles of interest in the range 15◦ to 90◦ off axis for LANSCE WNR, on axis for TSL ANITA. The simulations for which results are presented here employ a model without any target shielding.

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TABLE I: Model parameters [5], [9], [12] LANSCE target length target diameter proton energy range

70 30 800 20

ANITA 24 50 180 2.5

mm mm MeV m

III. S IMULATION RESULTS A. Neutrons We have compared our simulated neutron fields at LANSCE with independent calculations [13] and measurements [14], and our ANITA results with an analytical model derived from independent MCNPX calculations supported by time-of-flight measurements [5]. 1) LANSCE WNR: Fig. 1 compares our calculations of LANSCE neutron spectra to those from [13]. Uncertainties in the LANSCE calculations are around 1% across most of the energy range [13]; these are understood to represent standard counting uncertainties from Monte Carlo calculations. The corresponding uncertainties in our own calculations are below 3% across most of the energy range. The characteristic bimodal form of the spallation neutron spectrum is clearly visible, as is the strong forward preference of the prompt cascade peak around 100 MeV and the relative isotropy of the evaporation peak around 1 MeV. The results calculated with the binary model predict a lower neutron yield in the evaporation peak. In the cascade peak the binary and Bertini data agree well, with neutron yields above 10 MeV at 30◦ of 0.21 n p−1 sr−1 (Bertini) and 0.19 n p−1 sr−1 (binary). At 30◦ , corresponding to the position of the LANSCE ICE House [2], we also have measured data available [14]. These are compared with our own calculations and those from [13] in Fig. 1c. In order to make a direct comparison with calculations (flux in n p−1 sr−1 ), the measured data are normalized to give the same integral yield above 10 MeV as the results of our calculations (Bertini and binary averaged). For the data shown in Fig. 1c this implies an average proton current to target of 1.44 µA; allowing for interruptions to the beam this is consistent with the nominal proton current during the experiment (1.8 µA). Uncertainty in the measurements is understood to be around 5% [15]. Fig. 1 shows good agreement between our calculations and

Proc. RADECS 2013 PB-10L 2/4 2.0 Bertini binary calculation [13]

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normalized neutron yield

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Fig. 3: Integral neutron spectra at TSL ANITA

0.20 neutron yield /n p−1 sr−1

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Bertini binary calculation [13] measurement [14]

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1 neutron yield /n p−1 sr−1

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Fig. 2: Normalized neutron spectra at TSL ANITA. Normalization is to 1 n p−1 sr−1 above 10 MeV

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Fig. 1: Neutron spectra at LANSCE WNR Target 4

both measurement and LANSCE calculations. The binary data are close to the LANSCE calculations above about 50 MeV and below about 1 MeV, and represent the measured spectrum more closely than both the LANSCE calculations and the Bertini data across the energy range of interest. The Bertini data are closer to the LANSCE calculations between about 1 MeV and about 50 MeV, but both the Bertini data and

LANSCE calculations appear to overestimate the neutron yield in the evaporation peak, when compared with measurements. 2) TSL ANITA: Fig. 2 compares our calculations of TSL neutron spectra to the model given in [5]. The observation angle is 0◦ . The spectra in Fig. 2 are normalized to 1 n p−1 sr−1 above 10 MeV, showing the shapes of the curves unencumbered by considerations of absolute neutron yield. Once again, the Bertini data predict a greater yield in the evaporation peak, and the binary data correspond more closely to the independent model. The absolute neutron yield is somewhat less than that inferred from [5], however, as illustrated by the integral spectra in Fig. 3. The yield above 10 MeV calculated using the binary model is 0.029 n p−1 sr−1 , about 63% of that inferred from [5] (0.047 n p−1 sr−1 ). Uncertainty in the integral fluence rate in the ANITA model is understood to be about 10%. Statistical uncertainties in our Monte Carlo calculations of differential yield are below 6% for most of the energy range; the corresponding uncertainty in the integral fluence rate above 10 MeV is less than 1%. B. Photons We calculated prompt photon fields during the same simulations used to calculate the neutron fields described in

Proc. RADECS 2013 PB-10L 3/4

gamma yield /γ p−1 sr−1 MeV−1

1

rent (200 nA) to be 17 mSv h−1 (1.7 rem h−1 ). This is consistent with the upper limit of approximately 4 rem h−1 reported by Prokofiev et al. [5]. Although the photon yield (per unit solid angle) is around an order of magnitude greater at LANSCE than at TSL, the gamma fluence rate (per unit area) and dose rate experienced during irradiations are about an order of magnitude lower, because of the greater range to the standard irradiation position at the ICE House.

10−1 10−2 10−3 10−4 10−5 0.01

ICE House ANITA

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IV. D ISCUSSION 1 energy /MeV

10

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gamma yield /γ p−1 sr−1

1 10−1 10−2 10−3 10−4 0.01

ICE House ANITA

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Fig. 4: Photon spectra at LANSCE ICE House and TSL ANITA

section III-A. The results presented in this section are all from simulations using the binary intranuclear cascade model; results of the Bertini model were very similar with very slightly reduced photon yields. Fig. 4 shows differential and integral photon spectra at LANSCE ICE House and TSL ANITA. In each spectrum a peak at 0.5 MeV is visible, and attributed to electronpositron annihilation at 511 keV. The spectra are dominated by continua between about 100 keV and about 10 MeV. At LANSCE the continuum is very strong and overwhelms all but the annihilation peak. Above the annihilation peak the continuum decays approximately according to a power law with exponent a little greater than 2. In the ANITA beam the continuum is less intense and softer and several spectral lines are visible. At neither beam does the structure below about 500 keV contribute significantly to the integral photon fluence, as illustrated by the integral spectra in Fig. 4b. The E −1 component in the ANITA spectrum below 100 keV is an artefact of small counts in that region, corresponding to one photon count per bin (the bins are logarithmically spaced). Following [16], we calculate the gamma dose rate at the ANITA standard user position (2.5 m range) and operating cur-

Our calculations using Geant4 with the binary intranuclear cascade model provide excellent reproduction of independent calculations and measurements of the neutron spectra at both LANSCE ICE House and TSL ANITA. Our calculations match those provided by LANSCE very closely above about 100 MeV and below about 1 MeV. Although they match the LANSCE calculations less well between 1 MeV and 100 MeV they provide a good representation of the LANSCE measurements in this region at 30◦ . Geant4 calculations using the Bertini model agree closely with the LANSCE calculations above 1 MeV at angles from 15◦ forward to 90◦ off axis, however both those sets of calculations overestimate neutron yield in the evaporation peak when compared with measurements at 30◦ . We do not know the methodology used to provide the data in [13]; however, as this region of the neutron spallation spectrum is strongly dependent on intranuclear cascade processes, we think it likely that a Bertini model was used. The greatest discrepancy between our simulation results and measured data is in absolute neutron yield, which might be underestimated in our results, perhaps by as much as 40% at ANITA. This is not attributable to measurement uncertainties or statistical uncertainties in the Monte Carlo models. Our ANITA results, in particular, are likely to be influenced somewhat by our omission of shielding. The calculated photon spectra appear to be consistent with the blending of many emission lines from tungsten and with the presence of bremsstrahlung. Compared to the prompt gamma spectrum at LANSCE, that at ANITA has a lower yield, softer continuum, and visible discrete lines. This appears to be consistent with a greater number of reaction channels being available to the more energetic primary beam at LANSCE. V. C ONCLUSIONS We have demonstrated Geant4 models of neutron and photon fields at spallation sources used for single-event testing. Model validation against independent calculations and measurements show excellent reproduction of neutron spectra using the Geant4 QGSP BIC HP physics list, although our models might underestimate absolute neutron yield slightly. Our calculations have yielded what we think are the first calculations of prompt gamma spectra at neutron SEE test facilities, and are consistent with available experimental evidence of gamma dose. We predict that gamma fields are dominated by continua between about 100 keV and about

Proc. RADECS 2013 PB-10L 4/4 10 MeV, with integral yields approximately 0.26 γ p−1 sr−1 and 0.046 γ p−1 sr−1 at LANSCE ICE House and TSL ANITA, respectively. R EFERENCES [1] Measurement and Reporting of Alpha Particle and Terrestrial Cosmic Ray-Induced Soft Errors in Semiconductor Devices, JEDEC Std. JESD89A, Oct. 2006. [2] ICE House. Web site. Los Alamos National Laboratory. [Online]. Available: http://lansce.lanl.gov/NS/instruments/ICEhouse [3] E. Blackmore et al., “Improved capabilities for proton and neutron irradiations at TRIUMF,” in 2003 IEEE Radiat. Effects Data Workshop Record, 2003, pp. 149–155. [4] E. W. Blackmore, “Development of a large area neutron beam for system testing at TRIUMF,” in 2009 IEEE Radiat. Effects Data Workshop Record, 2009, pp. 157–160. [5] A. V. Prokofiev et al., “Characterization of the ANITA neutron source for accelerated SEE testing at The Svedberg Laboratory,” in 2009 IEEE Radiat. Effects Data Workshop Record, 2009, pp. 166–173. [6] L. H. Zhang et al., “In-situ neutron dosimetry for single-event effect accelerated testing,” IEEE Trans. Nucl. Sci., vol. 56, no. 4, pp. 2070– 2076, Aug. 2009. [7] L. H. Zhang and S. P. Platt, “Minimally invasive neutron beam monitoring for single-event effects accelerated testing,” in Proc. 12th European Conf. Radiat. Effects Compon. Syst., 2011, paper DW-30. [8] L. H. Zhang, “Neutron beam monitoring for single-event effects testing,” Ph.D. dissertation, University of Central Lancashire, 2012. [9] S. A. Wender and P. W. Lisowski, “A white neutron source from 1 to 400 MeV,” Nucl. Instr. Meth. Phys. Res. B, vol. 24-25, pp. 897–900, 1987. [10] S. Agostinelli et al., “Geant4–a simulation toolkit,” Nucl. Inst. Meth. Phys. Res., A, vol. 506, pp. 250–303, 2003. [11] J. Allison et al., “Geant4 developments and applications,” IEEE Trans. Nucl. Sci., vol. 53, no. 1, pp. 270–278, Feb. 2006. [12] A. V. Prokofiev, TSL, Sep. 2013, personal communication. [13] About WNR beam. Web page. Los Alamos National Laboratory. [Online]. Available: http://wnr.lanl.gov/newwnr/About/Beam.shtml [14] Unpublished measurements, 2005. [15] S. A. Wender et al., “A fission ionization detector for neutron flux measurements at a spallation source,” Nucl. Inst. Meth. Phys. Res., A, vol. 336, no. 1–2, pp. 226–231, Nov. 1993. [16] S.-G. Kwon et al., “Calculation of neutron and gamma-ray flux-to-doserate conversion factors,” J. Kor. Nucl. Soc., vol. 12, no. 3, pp. 171–179, Sep. 1980.