The neutron, gamma-ray, X-ray spectrometer (NGXS): A compact instrument for making combined measurements of neutrons, gamma-rays, and X-rays

Acta Astronautica 93 (2014) 524–529

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The neutron, gamma-ray, X-ray spectrometer (NGXS): A compact instrument for making combined measurements of neutrons, gamma-rays, and X-rays David J. Lawrence a,n, William C. Feldman b, Robert E. Gold a, John O. Goldsten a, Ralph L. McNutt a a b

Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA Planetary Science Institute, Tucson, AZ 85719, USA

a r t i c l e in f o

abstract

Article history: Received 22 December 2011 Received in revised form 5 June 2012 Accepted 21 June 2012 Available online 20 July 2012

The Neutron, Gamma ray, and X-ray Spectrometer (NGXS) is a compact instrument designed to detect neutrons, gamma-rays, and hard X-rays. The original goal of NGXS was to detect and characterize neutrons, gamma-rays, and X-rays from the Sun as part of the Solar Probe Plus mission in order to provide direct insight into particle acceleration, magnetic reconnection, and cross-field transport processes that take place near the Sun. Based on high-energy neutron detections from prompt solar flares, it is estimated that the NGXS would detect neutrons from 15 to 24 impulsive flares. The NGXS sensitivity to 2.2 MeV gamma rays would enable a detection of  50–60 impulsive flares. The NGXS is estimated to measure  120 counts/s for a GOES C1-type flare at 0.1 AU, which allows for a large dynamic range to detect both small and large flares. & 2012 IAA. Published by Elsevier Ltd. All rights reserved.

Keywords: Neutrons Gamma rays X-rays

1. Introduction The Neutron, Gamma ray, and X-ray Spectrometer (NGXS) is a compact instrument designed to detect neutrons, gamma-rays, and hard X-rays. The original goal of NGXS was to detect and characterize neutrons, gamma rays, and x-rays from the Sun as part of the Solar Probe Plus (SPþ) mission in order to provide direct insight into particle acceleration, magnetic reconnection, and cross-field transport processes that take place near the Sun via energetic solar flare processes [1]. In addition to measuring neutrons and gamma rays from large solar flare events, NGXS measurements could also be used to characterize new types of energetic solar events and understand the seed population of energetic particles [2]. To make the requisite observations on a highly mass-constrained mission, appropriate trades were made between the sensor size and

n

Corresponding author. E-mail address: [email protected] (D.J. Lawrence).

mass versus the types of measurables, their energy ranges, and detection sensitivity. This paper describes the design and implementation of the NGXS as well as its detection sensitivity in an environment rich in energetic particles.

2. NGXS overview The NGXS instrument is configured as two sensor heads that operate independently while sharing a common electronics box (Fig. 1). The Neutron Spectrometer (NS) and Gamma-Ray Spectrometer (GRS) are combined to enable coincidence techniques that actively reject background charged particles. Effective background rejection is critical because flare-produced neutrons and gamma rays will often be accompanied by large fluxes of energetic ions and electrons. The GRS detector is embedded within the NS detector to achieve almost full 4p active shielding; only those events in anti-coincidence (AC) with the NS detector are accepted. Background

0094-5765/$ - see front matter & 2012 IAA. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actaastro.2012.06.017

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rejection for the NS is accomplished by identifying neutrons with a time-correlated two-pulse sequence in coincidence with an associated gamma ray detected in the GRS—an effective triple-coincidence measurement. The NS/GRS detector combination is read out with a single photomultiplier tube (PMT) in a phoswich arrangement that maximizes the sensitive detector volume while minimizing the amount of attenuating materials and photodetector mass. ‘Phoswich’ is a shortened term for phosphor sandwich, and refers to scintillator light readout arrangements where multiple scintillators are read out with a single PMT and signal separation is achieved via pulse time and shape discrimination [3]. A separate CdTe X-ray Spectrometer (XRS) provides high-energy resolution measurements of the hard X-ray region down to the low-energy limit of 20 keV. The estimated total mass of the NGXS is 2.6 kg, and its estimated power is 3 W. The inner GRS sensor uses a bismuth germanate (BGO) scintillator (+3.8  3.8 cm long) similar to GRS instruments flown on the Near Earth Asteroid Rendezvous (NEAR), Lunar Prospector (LP), and Dawn missions [4–6]. BGO is well understood and offers the highest photopeak efficiency of any scintillator, which is critical when measuring short-duration bursts of high-energy gamma rays. Its energy resolution of 6% at 2 MeV [7] is more than adequate to resolve the 2.2 MeV H neutron capture line. The GRS covers 0.1–10 MeV with 512 energy channels. Both integral (all events) and AC spectra are reported. The outer NS sensor uses a +9.7  9.7 cm long wellshaped boron-loaded scintillator (BC-454) similar to those used successfully on the LP, Mars Odyssey (MO), Dawn, and MESSENGER neutron instruments [5,6,8,9]. Borated plastic scintillators are sensitive to fast neutrons (0.5–20 MeV) and produce a unique two-pulse sequence. An initial prompt pulse caused by energy loss of recoil protons in the scintillator provides a measure of the incident neutron energy with an energy resolution o50%. A delayed neutron capture pulse unambiguously identifies a neutron event via its energy from the 10B(n,a) reaction along with its 2 ms characteristic time delay from the prompt pulse. Detection of both valid pulse types provides a natural means of rejecting charged particles and locally induced gamma rays. Additional background rejection is achieved by further requiring detection of the coincident 478 keV 10B(n,a) gamma-ray in the GRS, as has been demonstrated with the LP and MESSENGER GRS instruments. Signals from the NS and GRS sensors are both measured with a single PMT and the risetime difference between the fast BC-454 and slow BGO scintillators allows straightforward digital electronic separation of the signals. Excellent signal separation has been demonstrated in laboratory tests using neutrons and electrons with prototype sensors (Figs. 1 and 2). For incident neutrons below 0.5 MeV, the NS counts the number of capture-only type interactions, which provides a measure of lower energy primary neutrons as well as neutrons that down-scatter in the spacecraft before striking the detector. The XRS detector, optimized to cover the 20–200 keV energy range, is a pre-packaged assembly similar to the Si-PIN detector flown on NEAR. The 3  3  1 mm3 CdTe detector is mounted on a two-stage thermoelectric cooler

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Fig. 1. Views of the combined NS/GRS sensor and the miniature XRS sensor showing active materials. The GRS detector (BGO) is surrounded by the NS detector (BC-454) to achieve almost full 4p shielding against charged particles. The single PMT readout maximizes detector volume and minimizes attenuating materials. A passive collimator that sits atop the XRS excludes background outside the solar disk.

Fig. 2. Laboratory results demonstrate clean separation of gamma ray and neutron signals from prototype sensor using pulse shape discrimination. Phoswich signal fraction is defined as the ratio of a short signal integration to a long signal integration and discriminates between the fast plastic signals that are dominated by neutrons and slow BGO signals that are dominated by gamma-rays.

(TEC) in a sealed vacuum can with a beryllium window. An external collimator made of copper tungsten strongly suppresses the charged-particle background by restricting the field-of-view (FOV) to the solar disk at 9.5 Rs, along with margins for pointing and alignment. 3. Neutron sensitivity calculations 3.1. Neutron fluences We scale measured neutron count rates to the neutron fluences calculated by Hua and Lingenfelter [10] (hereafter abbreviated H and L) and Murphy et al. [11] (Fig. 3). These studies provide energy dependent fluences in units 1 of (neutrons ster  1 MeV  1 Np ), where Np indicates the number of flare protons greater than 30 MeV. We note that these fluences assume the neutrons are created in an

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Fig. 3. Neutron fluences used in this study as calculated for Np ¼ 1030 and at a distance of 1 Rs.

impulsive flare scenario where the emitted neutrons are created on the Sun from an impulse instead of over an extended period of hours. However, this study does not account for other neutron-generating events on the Sun that may scale differently than calculated by the above authors. To convert these fluences of H and L [10] and Murphy et al. [11] into more convenient units, we scaled them to Np ¼1030, and to neutrons cm  2 MeV  1 using the factor: 1030 =4pR2S ¼ 1:6  107 , for Rs ¼7  1010 cm. Fig. 3 shows the fluence spectrum for three different examples from H and L [10] and Murphy et al. [11]. The total fluences per steradian for Np ¼1030 at the solar surface for the three different fluence models are: FTot, H and L, Fig.10 ¼1.0  1026 n ster  1; FTot, H and L, Fig. 8b ¼7.2  1026 n ster  1; FMurphy 26 n ster  1. According to Shih et al. [12], a et al. ¼ 2.9  10 flare with Np ¼1030 is roughly equivalent to an M7 flare. To determine the total neutron counts as a function of energy and distance, the fluence at one Rs needs to be propagated to different distances as a function of energy. The fluence from Fig. 3 needs to be multiplied by two factors: (1) Rs2/D2; and, (2) e-(t/t) where t is the propagation time for a neutron with a given energy E to travel from the Sun to D, and t is the neutron mean life of 886 s. Fig. 4 shows these propagated fluences for the H and A, Fig. 8b fluence given in Fig. 3. There is a significant drop off for lower energy neutrons due to the exponential factor and an additional drop off for the 1/D2 factor. 3.2. Number of flares per flare size Shih et al. [12] provide data for 26 solar flares measured by RHESSI, where they measured the flux of the 2.2 MeV neutron capture line. With these measurements, they used model calculations to estimate Np for each of those flares. Fig. 5 shows the integral number of flares greater than Np, INp , (where Np is scaled in units of 1030 protons with energy greater than 30 MeV) and scaled by a factor of 2(7/4) to account for the seven year SPþ mission compared to the four years of RHESSI data, as well as a factor of two to account for the fact that RHESSI only sees the Sun for 50% of its observing time. With the fit of INp to

Fig. 4. Neutron fluences propagated to distances 9–215 Rs from the surface of the Sun.

Fig. 5. Integral number of flares greater than Np as a function of Np. over the life of the Solar Probe mission.

log (Np), the differential number of flares in a log unit of Np is 9.46 flares per log (Np). No account has been made for varation of flare occurrence with solar cycle. 3.3. Probability to observe flares Assuming a 2015 launch date, Fig. 6 shows the SPþ spacecraft calculated time per distance bin, D, where the D bins start at 9 Rs and have a width of 6 Rs out to 213 Rs. The maximum amount of time is spent around 150 Rs, which is the orbit of Venus. With the times calculated in Fig. 6, and knowing the number of flares per log (Np) bin per hour, NFlare,hour(Np)¼ 9.46 flares per log (Np)/(7 years  365  24) ¼1.54  10  4 flare per hour per log (Np) bin, the probable number of detected flares in a given log (Np) bin as a function of distance D is: P(D,Np)¼NFlare,hour  TimeRs. Fig. 6 shows this probable number of flares as a function of Rs. 3.4. NS efficiency Fig. 7 shows a plot of efficiency versus energy for detector models and measurements. The black data points

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using the relation Z 50 MeV C Total ðD,Np Þ ¼

EI ¼ 1:5 MeV

Fig. 6. SP þtime spent in a given Rs bin as a function of distance from the Sun (left axis) and corresponding probable flare detections in a given log (Np) bin size (right axis).

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Np eA ðEI ÞFðEI ,DÞdEI þ C back

where F(EI,D) is the energy dependent fluence as a function of distance, D, from the spacecraft to the Sun (here using the H and L Fig. 8b fluence), and eA is the incident, energy dependent efficiency area product of the neutron sensor. The left part of Fig. 8 shows the total counts versus D for flares of various sizes and are color-coded based on the probability of detecting a given flare. The total background counts are Cback ¼cbackDt, where cback is the background counting rate, and Dt corresponds to the interval of time elapsed between the arrival of the high-energy neutrons of 50 MeVand the arrival of low energy neutrons of 1.5 MeV. For the MESSENGER detector, cback  0.5 cps for En 4 1 MeV [2]. Here, we scale the MESSENGER background to the smaller sized NGXS neutron sensor. The dashed line in Fig. 8 (left) shows the total background counts as a function of distance for energies from 1.5 to 50 MeV. To determine if a flare is detected, the signal-to-noise, Sn(D,Np) is calculated using the following: Sn ðD,Np Þ ¼

C Total ðD,Np ÞC Back ðD,N p Þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C Back ðD,Np Þ

Fig. 8 (right) shows a contour plot of Sn(D,Np). A 3s detection (i.e., signal-to-noise of three) can be made for a flare size of 0.3  1030 at the closest distance of 10 Rs. As a summary plot, Fig. 9 shows the detected number of flares versus detector volume, V. For larger volume sensors, there are an increasing number of detected flares, but it only scales as V0.24, which is a relatively weak dependence. For the SPþ detector, the detected number of flares is 24. For the fluence model of Murphy et al. [11], the total number of detected flares is 15. Fig. 7. Plot of efficiency versus energy for various detector models and configurations.

show an MCNPX calculation of the efficiency for 10B(n,a) capture reactions in the MESSENGER NS. The circles show measured efficiencies from Drake et al. [13] for a BC454 rod that has the same volume as the MESSENGER NS, but different shape. Using MCNPX calculations, we have determined that the fast neutron efficiency scales as the volume of the sensor. Therefore, the measurements should be a reasonable check on the modeled values. The measurements of [13] show a similar shape with the modeled values, but are low by a factor of two. We do not yet have a full understanding of the reason for this discrepancy, so to be conservative, we use the MCNPX calculated efficiencies divided by two to match the values from [13].

4. Gamma-ray sensitivity calculations We calculate the gamma-ray sensitivity in a similar way as was done for the neutrons. For the GRS, we used a standard calculated efficiency of 0.2 at 2.2 MeV. The area of the detector is 11.3 cm2. Thus, the effective area is 2.26 cm2. For background counts, we scale from Lunar Prospector cruise data [7], which was also a space-based BGO sensor (Fig. 10). The size of the LP-GRS is 54 cm2, with an efficiency at 2.2 MeV of  0.4. The scaling to the SPþ BGO sensor is then (0.4  54)/(0.2  11.3) ¼9.5. During the LP cruise, the counting rate at 2.2 MeV is 3.67 cps. The equivalent background counting rate for the SPþ sensor is 0.38 cps. From [12], the estimated number of counts in the SPþGRS can be determined using C Total ¼

3.5. Total counting rate, signal-to-noise, and total detected flares With all the above information, the total neutron counts, as a function of flare size, can be determined

n2:2 eAD2o Dt D2

where n2.2 is the number of photons per cm2 from [12], e is the sensor efficiency, A is the sensor area, Dt is the time over which the total fluence is measured, D is the spacecraft distance from the Sun, and Do is 1 AU, where

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Fig. 8. (Left) Total counts versus spacecraft distance from the Sun. Each line shows the number of counts for a flare size in terms of number of protons, Np, greater than 30 MeV ranging from 0.03 to 8000 in units of 1030. The color-coding shows the probability of detecting a flare during the seven year SP þ mission. The dashed orange line shows the total background counts for neutron energies from 1.5 to 100 MeV, assuming a background counting rate of 0.5 cps. (Right) Contour plots of signal-to-noise as a function of flare size versus spacecraft-to-Sun distance.

the RHESSI measurements were made. As an intermediate Dt given in [12], we used 1 h. When the total counts and probabilities are added up, there is a total of 50–60 flares that would be detected using the assumptions stated above. 5. X-ray sensitivity calculations

Fig. 9. Number of detected neutron flares versus sensor volume.

For the XRS, we scaled the dynamic range to have sensitivity for a wide range of flare sizes ranging from moderately small C1 to large M and X class flares. The driving requirement for this dynamic range is obtaining statistically significant sensitivity for the smaller flares. We determine the sensitivity of the XRS using RHESSI data as reported by Saint-Hilaire et al. [14] from 172 hard X-ray peaks during 53 solar flares that exhibited a double-footprint structure. Saint-Hilaire et al. [14] found a correlation between a solar flare maximum GOES 1–8 A˚ flux and the total hard X-ray flux F 50 ¼ AF aGOES where A¼4.7  103 and a ¼ 0.8. For a C1 flare, FGOES ¼10  6, so that F50 ¼0.0745 photons cm  2 s  1 keV  1. Also from [14], a typical spectral index for hard X-rays is g ¼3.3, where FX-ray ¼ CE  g. Setting FX-ray ¼F50, C¼ 3  104. Integrating the X-ray flux from 20 to 200 keV, one obtains 200 CEg þ 1  photons F Total ¼  ¼ 13:2 g þ 1  cm2 s 20

If we assume 100% efficiency for the 3  3 mm2 CdTe detector from 20 to 200 keV, we obtain a counting rate of 1.18 cts/s. Finally, we factor in the distance from 1 to 0.1 AU, and we get a total of 118 cts/s for energies from 20 to 200 keV for a C1 flare at 0.1 AU. 6. Summary Fig. 10. Counting rate of reaching the Moon. The 2.2 MeV gamma-ray line. for the SPþ configuration,

the LP-GRS during the 3-day cruise prior to vertical lines show the region around the The total counting rate is 3.67. After scaling the equivalent counting rate is 0.38 cps.

Based on high-energy neutron detections from prompt solar flares, it is estimated that the NGXS would detect neutrons from 15 to 24 impulsive flares. The NS would

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have more than doubled the total number of detected neutron flares from all space-based missions to date [15], as well as detected extended neutron events similar to the type observed by the MESSENGER NS on 31 December, 2007 [2]. The GRS sensitivity to 2.2 MeV gamma rays would enable a detection of  50–60 impulsive flares. The XRS is estimated to measure 120 counts/s for a GOES C1-type flare at 0.1 AU, which allows for a large dynamic range to detect both small and large flares. While originally designed for the SP þmission, the NGXS can provide robust measurements of neutrons, gamma rays, and X-rays for a wide variety of mass constrained mission scenarios where charged particle rejection is needed. Acknowledgements The authors thank two anonymous reviewers for detailed and helpful reviews. This work was supported by internal development funding from JHU/APL. References [1] Solar Probe Plus: Report of the Science and Technology Definition Team, NASA/TM-2008-214161, 2008. [2] William C. Feldman, et al., Evidence for extended acceleration of solar-flare ions from 1–8 MeV solar neutrons detected with the MESSENGER neutron spectrometer, J. Geophys. Res. http://dx.doi.o rg/10.1029/2009JA014535.

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