A comparative study of LaBr 3 (Ce 3+ ) and CeBr 3 based gamma-ray spectrometers for planetary remote sensing applications

Transcription

1 A comparative study of LaBr 3 (Ce 3+ ) and CeBr 3 based gamma-ray spectrometers for planetary remote sensing applications A. Kozyrev,, I. Mitrofanov, A. Owens, F. Quarati, J. Benkhoff, B. Bakhtin, F. Fedosov, D. Golovin, M. Litvak, A. Malakhov, M. Mokrousov, I. Nuzhdin, A. Sanin, V. Tretyakov, A. Vostrukhin, G. Timoshenko, V. Shvetsov, C. Granja, T. Slavicek, and S. Pospisil Citation: Review of Scientific Instruments 87, (2016); doi: / View online: View Table of Contents: Published by the American Institute of Physics

2 REVIEW OF SCIENTIFIC INSTRUMENTS 87, (2016) A comparative study of LaBr 3 (Ce 3+ ) and CeBr 3 based gamma-ray spectrometers for planetary remote sensing applications A. Kozyrev, 1,a) I. Mitrofanov, 1 A. Owens, 2 F. Quarati, 3,4 J. Benkhoff, 2 B. Bakhtin, 1 F. Fedosov, 1 D. Golovin, 1 M. Litvak, 1 A. Malakhov, 1 M. Mokrousov, 1 I. Nuzhdin, 1 A. Sanin, 1 V. Tretyakov, 1 A. Vostrukhin, 1 G. Timoshenko, 5 V. Shvetsov, 5 C. Granja, 6 T. Slavicek, 6 and S. Pospisil 6 1 Space Research Institute of the Russian Academy of Sciences (IKI), 84/32 Profsoyuznaya St., Moscow , Russia 2 European Space Agency, ESTEC, Keplerlaan, 2200 AG Noordwijk, The Netherlands 3 AP, RST, FAME, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands 4 Gonitec BV, J. Bildersstraat 60, 2596 EJ Den Haag, The Netherlands 5 Joint Institute for Nuclear Research, Joliot-Curie 6, Dubna, Moscow Region , Russia 6 Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, Prague 2, Czech Republic (Received 1 December 2015; accepted 4 July 2016; published online 12 August 2016) The recent availability of large volume cerium bromide crystals raises the possibility of substantially improving gamma-ray spectrometer limiting flux sensitivities over current systems based on the lanthanum tri-halides, e.g., lanthanum bromide and lanthanum chloride, especially for remote sensing, low-level counting applications or any type of measurement characterized by poor signal to noise ratios. The Russian Space Research Institute has developed and manufactured a highly sensitive gamma-ray spectrometer for remote sensing observations of the planet Mercury from the Mercury Polar Orbiter (MPO), which forms part of ESA s BepiColombo mission. The Flight Model (FM) gamma-ray spectrometer is based on a 3-in. single crystal of LaBr 3 (Ce 3+ ) produced in a separate crystal development programme specifically for this mission. During the spectrometers development, manufacturing, and qualification phases, large crystals of CeBr 3 became available in a subsequent phase of the same crystal development programme. Consequently, the Flight Spare Model (FSM) gamma-ray spectrometer was retrofitted with a 3-in. CeBr 3 crystal and qualified for space. Except for the crystals, the two systems are essentially identical. In this paper, we report on a comparative assessment of the two systems, in terms of their respective spectral properties, as well as their suitability for use in planetary mission with respect to radiation tolerance and their propensity for activation. We also contrast their performance with a Ge detector representative of that flown on MESSENGER and show that: (a) both LaBr 3 (Ce 3+ ) and CeBr 3 provide superior detection systems over HPGe in the context of minimally resourced spacecraft and (b) CeBr 3 is a more attractive system than LaBr 3 (Ce 3+ ) in terms of sensitivities at lower gamma fluxes. Based on the tests, the FM has now been replaced by the FSM on the BepiColombo spacecraft. Thus, CeBr 3 now forms the central gamma-ray detection element on the MPO spacecraft. Published by AIP Publishing. [ I. INTRODUCTION The recent introduction of cerium-activated lanthanum halide scintillators 1 has led to a renaissance in scintillation based gamma-ray spectrometry. In addition to their superlative scintillation properties (high light output, fast time response, and good stopping power), they have significant benefits when designing gamma-ray detection systems for resource-limited spacecraft not only in terms of reduced demands on power, mass, and accommodation, but also in terms of radiation hardness, reliability, and robustness, when compared to other technologies (e.g., HPGe). 2 The Mercury gamma-ray and Neutron Spectrometer (MGNS) 3 to be flown on ESA s BepiColombo mission to a) kozyrev@mx.iki.rssi.ru Mercury, were originally designed to accommodate a gammaray spectrometer based on LaBr 3 (Ce 3+ ) as its main detection element. However, while they have many advantages, all lanthanum halides suffer from one serious disadvantage, namely the high internal activity ( 1 Bq cm 3 ) resulting from the decay of 138 La which comprises 0.09% of naturally occurring stable lanthanum ( 139 La). LaBr 3 (Ce 3+ ) actually belongs to a wider group of rare-earth metal tri-halides of which CeBr 3 is of particular interest in that it still has good resolving power (typically 4% 5% as opposed to 3% 4% for LaBr 3 (Ce 3+ )) but virtually no naturally occurring radioactive isotopes. 4 Consequently, it has a much lower internal background than LaBr 3 (Ce 3+ ) and as such, a much better sensitivity across much of the nuclear transition energy range. In fact, if external sources of background can be neglected (e.g., the omnipresent natural radioactivity, i.e., the lab background), the internal CeBr 3 crystal background lowers by an average factor of /2016/87(8)/085112/8/$ , Published by AIP Publishing.

3 Kozyrev et al. across the energy band 100 kev 5 MeV and a factor of 16 at 1461 kev the signature for 40K, an important tracer of planetary evolution. However, in the laboratory or space, these factors will be less due to the diluting effects of external background (e.g., natural environmental radioactivity, cosmicrays, and secondary production from the spacecraft). Recently, technical improvements in melt growth driven by ESA have led to the production of large crystals of CeBr3. To assess the competitiveness of CeBr3 as compared to LaBr3 (Ce3+), the 3-in. LaBr3(Ce3+) crystal in the MGNS FSM was retrofitted with a 3-in. CeBr3 crystal and subjected to fullflight qualification tests. In this paper, we present the results of comparative performance tests between the flight model (FM) (which still contains a 3-in. LaBr3(Ce3+)) and the flight spare model (FSM). Note that except for the crystals, the two models can be considered identical. For comparison, we also carried out the same tests on a 300 cm3 high-purity Ge detector. II. THE MERCURY GAMMA-RAY AND NEUTRON SPECTROMETER (MGNS) The MGNS was developed in the Russian Space Research Institute for use onboard the Mercury Polar Orbiter (MPO) as the Russian Federal Space Agency s science contribution to ESA s BepiColombo mission.5 The instrument3 is specifically designed to analyze the chemical composition of Mercury s subsurface using both their gamma-ray and neutron signatures and is in fact comprised of two subsystems, a gamma-ray spectrometer (GRS) and a separate neutron spectrometer (NS) (see Figure 1). The signatures these spectrometers receive are generated by cosmic-rays that continually bombard Mercury s Rev. Sci. Instrum. 87, (2016) surface which are then remotely sensed by the spectrometers while the spacecraft is orbiting the planet. Remote sensing is an established technique for determining elemental compositions and has played a significant role in the exploration of airless (or near airless) solar system bodies, such as the Moon, Mars, and the large S-class asteroid Eros.6 8 The gamma-ray measurements give information on elements located at a depth of 30 cm below the surface of the regolith, whereas neutrons give information on layering and the spatial distribution of rare-earth elements and hydrogenous materials to a depth of 1.5 m. By combining these data, it is possible to differentiate between the regolith, soil, and buried components, such as water. Gamma-ray maps can also be added to conventional imagery to make correlations with geophysical structures more readily apparent. The gamma-rays themselves arise primarily from two sources: (1) cosmic-ray induced gamma-rays from the planetary surface and near surface and (2) gamma-rays emitted spontaneously by primordial naturally occurring radionuclides like K, Th, and U in the rocks and soils that comprise the surface regolith. In the first case, incident cosmic rays interact with the nuclei of the regolith producing a copious amount of secondaries including neutrons. Low energy neutrons can then be captured by the nuclei in the regolith which then de-excite by producing gamma-rays, while the fast neutrons produce gamma-rays by inelastic scattering-off the subsurface nuclei. These nuclei then de-excite by emitting characteristic gamma-rays of soil constituting elements; e.g., at 0.85 MeV (Fe), 1.78 MeV (Si), 2.22 MeV (H), 4.43 MeV (C), and 6.13 MeV (O). These gamma-rays can then go to Compton scatter to lower energies, be absorbed, or (provided the depth of interaction is not too great, 30 cm) can then escape and FIG. 1. (a) Schematic of the flight spare MGNS showing a cross-sectional view of the gamma-ray spectrometer (GRS) NS refers to the neutron spectrometer. (b) A functional block diagram of the GRS.

4 Kozyrev et al. Rev. Sci. Instrum. 87, (2016) leak into space where they can be detected either by an orbiting spectrometer or by spectrometer on-board a lander. Numerical calculation 9 indicates that the planetary surface will emit a forest of nuclear lines in the energy range MeV, stressing the importance of high sensitivity of the detection system up to high energies. A. The gamma-ray spectrometer (GRS) The MGNS instrument consists of neutron and gammaray spectrometers. The neutron spectrometer is based on the HEND instrument design 10 which flew successfully on Mars Odyssey since The gamma-ray spectrometer (GRS) of the Flight Spare Model (FSM) is presently based on a 77 mm diameter, 78 mm thick circular cylinder of CeBr 3 viewed by a Hamamatsu R , 76 mm diameter Photo Multiplier Tube (PMT) as is shown schematically in Fig. 1(a). A functional block diagram of the overall GRS system is shown in Fig. 1(b). The physical and mechanical properties of the new CeBr 3 crystal and the original LaBr 3 (Ce 3+ ) crystal are almost identical, and thus, the LaBr 3 (Ce 3+ ) crystal in the FSM could be replaced with no design changes to the mechanics and electronics of the GRS. In accordance with BepiColombo spacecraft requirements, both the FM and FSM of the MGNS were subjected to, and passed through, shock, vibration, and thermo-vacuum environmental tests. For the latter tests, the scintillation block was temperature cycled four times over the range ( 35 to +55) degrees at a rate of 12 /h. In Table I, we summarize the characteristics of the two models. The energy range of both models is 100 kev 9 MeV. Both models have a total mass of 5.5 kg and consume 6.44 W TABLE I. Characteristics of both the flight model (FM) fitted with a 3-in. LaBr 3 (Ce 3+ ) crystal and the flight spare model (FSM) fitted with a 3-in. CeBr 3 crystal. Parameter MGNS flight model MGNS flight spare model Gamma-ray detection element LaBr 3 (Ce 3+ ) CeBr 3 Readout element Hamamatsu R Energy range 100 kev 9 MeV FWHM energy resolution, E/E 3.9% at 662 kev 4.7% at 662 kev Total field of view 3 π sr Temporal resolution 1 20 s Intrinsic background 0.7 Bq cm Bq cm 3 (100 kev 9 MeV) Minimum detection limits to cm 2 s cm 2 s 1 gamma-ray line at 1 MeV (see Sec. IV C for details) Minimum detection limits continuum sources at 1 MeV (see Sec. IV C for details) cm 2 s cm 2 s 1 Surface spatial resolution 400 km Surface depth resolution for 30 cm gamma-ray Mass (kg) TLM data rate MB/day Total power 6.44 W of power. The only tangible changes are in the measured energy resolutions at 662 kev (3.9% FWHM for the FM and 4.7% FWHM for the FSM) and in the intrinsic background levels (0.7 Bq cm 3 for the FM and 0.01 Bq cm 3 for the FSM). To verify and compare the physical properties of both the FM and FSM of both crystals, we performed a number of measurements described in Sec. III. III. GAMMA-RAY MEASUREMENTS Gamma-ray measurements were carried out using the wide energy range, portable gamma-ray calibration station developed by the Czech Technical University. 12 The source was developed specifically to test and calibrate gamma-ray spectrometers over the entire nuclear transition energy region in a reasonably short time (minutes rather than hours). Normally line generating mechanisms at the highest energies are extremely inefficient and thus in wide energy range calibrations the time to complete a calibration test is driven by the highest energy lines and can be prohibitively long. The portable gamma-ray source is designed in such a way that the difference in intensities between the lowest and highest energies is minimized (less than two orders of magnitude) so that the exposure time to achieve a particular significance in a line flux at the highest energies can be achieved in a realistic time, while not incurring significant dead-time due to the low energy lines. The station consists of a 50 cm Plexi-glass cube which is filled with about 100 l of distillated water (H 2 O) and 22 kg of dissolved conventional (edible) salt (sodium chloride). At the center of the cube is a n/s 252 Cf neutron source which produces a Maxwellian shaped neutron energy distribution. 13 The neutrons are then moderated by the water which fills the cube. Since the mass of the hydrogen atoms forming the water is equal to that of the neutron, about half the neutron energy is lost per collision with a hydrogen nucleus, which when coupled with high (n,p) elastic scattering cross-section is very effective in moderating fast neutrons. The resulting moderated neutron spectrum peaks at thermal energies and is flat out to a few MeV. With appropriate and interchangeable targets the source can generate discrete gamma-ray spectra with energy up to 9.8 MeV. For the measurements described here, the chlorine in the salt forms the target. Discrete gamma-rays can then be generated by two mechanisms: (1) thermal neutron capture on the chlorine nuclei, i.e., 35 Cl(n,γ) 36 Cl, and (2) the inelastic scattering of fast neutrons by the chlorine nuclei, 35 Cl(n,n γ) 35 Cl. The gamma-ray spectrum generated contains many discrete lines, conveniently distributed evenly over the entire energy range. The principal gamma-ray lines detected and analyzed in the present work are those generated by neutron captures in 35 Cl as reported in Table II. The geometry of the measurements described here is illustrated in Figs. 2 and 3. For the flight-version detector, the minimum measuring times for calibration, i.e., the minimum time needed to collect at least counts in the full energy peak, was between 2 and 10 min (up to 6.2 MeV) and 20 and 30 min (up to 8 MeV) when the detector was placed at a distance 2 m from the station.

5 Kozyrev et al. Rev. Sci. Instrum. 87, (2016) TABLE II. Principle gamma-ray lines generated by the portable gamma-ray calibration station and used in the analysis. Source Isotope Energy (kev) Type of reactions a Annihilation Chlorine 35 Cl 786 NC 1165 NC 1955 NC Hydrogen H 2223 NC Potassium 40 K 1461 NO Thallium 208 Tl (Th) 2614 NO Chlorine 35 Cl 2864 NC 3062 NC 5600 SE of 6111 and DE of NC and SE of NC 6903 SE of NC 7790 NC 8068 SE of NC a NC neutron capture; NO naturally occurring; IS inelastic scattering; SE single escape; DE double escape. IV. RESULTS AND DATA ANALYSIS Fig. 4(a) shows the measured spectra obtained with the two MGNS modules: the FM based on LaBr 3 (Ce 3+ ) and the FSM based on CeBr 3, along with the spectrum obtained with the reference HPGe gamma-ray spectrometer. The measurement time for the FM was 369 min and for the FSM 885 min. The gamma-ray fluxes emitted by the portable gamma-ray calibration station and the corresponding count rate spectra recorded by the CeBr 3 crystal were simulated using the MC- NPX Monte-Carlo code 14 supported by the ENDF/B-VII.0 neutron and ENDF/B-VI photo-atomic data libraries. The results are shown in Fig. 4(b). Note that the walls, floor, and ceiling of the laboratory, as well as any secondary radiation emanating from them, were not included in the simulation, since they are not a subject of the present study. To compare both crystals we have estimated the relative detection sensitivity factor which we define for each gammaray line as the ratio of the signal-to-noise of a detector based on the CeBr 3 crystal to signal-to-noise of the detector based on the LaBr 3 (Ce 3+ ) crystal. The signal in this case are the source counts in the line and the noise is the root of the square of the number of background counts upon which the line sits. The energy dependent, relative detection sensitivity factors are presented in Fig. 5. In the energy range up to 3 MeV, the average value of the sensitivity factor is 1.7. Around 1.4 MeV, corresponding to the geologically important 40 K gamma-ray line energy, the comparative factor reaches 3.1. These results suggest that the CeBr 3 crystal is superior for detecting gammaray lines in the energy range up to 3 MeV and especially considering the 40 K gamma-ray line at 1460 kev. A. Energy resolution The FWHM energy resolutions of all three detectors were measured at a variety of energies and are shown in Fig. 6. It was found that for both LaBr 3 (Ce 3+ ) and CeBr 3 the energy resolution can be well fit by a power law of index 0.6, and can be expressed as E = 0.135R(%)E 0.6 (kev), (1) where R(%) is the fractional energy resolution, E/E, at 662 kev expressed as a percentage. For the present measurements R(%) = 3.9 for the LaBr 3 (Ce 3+ ) detector and 4.7 for the CeBr 3 detector. Also shown in Fig. 6 is the measured energy resolution for the HPGe detector. Following the prescription given in Owens, 15 the data were found to be well fit by a FIG. 2. Schematic illustrating the measurement geometry using the gamma-ray station.

6 Kozyrev et al. Rev. Sci. Instrum. 87, (2016) FIG. 3. Physical setup of the gamma-ray line measurements with the gamma-ray station. The measurements with the LaBr3(Ce3+), CeBr3, and HPGe detectors were performed at the same location to ensure identical gamma-ray background conditions. function of the form E = a1 E + a2 E 2 + a3 (kev), (2) where a1, a2, a3 are empirical constants determined by best fitting. For the present data, a1 = kev, a2 = , and a3 = 1.37 kev2. B. Radiation hardness and activation Radiation damage and activation in the space environment is almost entirely due to protons, which can be sepa- FIG. 4. (a) Wide-band gamma-ray spectra measured with the CeBr3 (blue line), LaBr3(Ce3+) (red line), and HPGe (black line) spectrometers, using the gamma-ray calibration station. The acquisition times for the FM, FSM, and HPGe were 369, 885, and 882 min, respectively. (b) A Monte Carlo simulation of the gamma-ray spectra produced by the portable gamma-ray calibration station is shown by the black line. The corresponding simulated count rate spectra recorded by the CeBr3 crystal is shown by the blue line. rated into two components: those due to the galactic cosmic rays and those originating in solar proton events (SPE s). The galactic cosmic-rays represent a steady-state low level activation component that is modulated by the 11 yr solar cycle. SPE s on the other hand can vary by many orders of magnitude. Events can last for days and fluxes can vary by as much as six orders of magnitude.16 Peak fluxes can stay high for periods of hours. Particle spectra tend to be flat, rolling over at about 30 MeV, but providing significant fluxes up to a few hundred MeV. Radiation damage due to fast neutrons can be ignored to first order, since they do not occur naturally in space but are produced as a consequence of secondary proton induced interactions in the detector and spacecraft. Similarly activation produced by secondary thermal neutrons can also be ignored since the average thermal neutron capture cross-sections in Ce, La, and Br are of the order of barns and even the integral resonant capture cross sections are only 12.1 b (La), 0.6 b (Ce), and 50 b (Br).17 FIG. 5. Relative detection sensitivity factors (defined to be the ratio of the signal-to-noise) of an MGNS measured with the CeBr3 detector to the signal-to-noise of an MGNS measured with the LaBr3(Ce3+) detector, as a function of energy.

7 Kozyrev et al. Rev. Sci. Instrum. 87, (2016) of protons/cm 2. An excess of counts is observed between 150 kev and 200 kev in CeBr 3, but overall the two spectra are very similar. Given there are no important planetary gamma-ray lines expected in the 150 kev 200 kev energy range (e.g., see Refs. 9 and 22), we conclude that proton activation of CeBr 3 is essentially the same as that observed in LaBr 3 (Ce 3+ ). The gamma-ray signatures dominating the intrinsic background of both scintillators are that of 77 Kr and 79 Kr. These nuclides are produced by Br + p nuclear reactions, e.g., 79 Br(p,n) 79 Kr. The measured total activity of CeBr 3 was found to decays with two main time constants: a fast one of about 20 h and a slower one of about 1500 h. The 79 Kr half-life of 35 h is compatible with the observed fast decay activation component. FIG. 6. The measured and fitted FWHM energy resolutions for the LaBr 3 (Ce 3+ ) detector, the CeBr 3 detector, and the reference HPGe detector. By repeating the experiments previously performed for LaBr 3 (Ce 3+ ) and reported in Ref. 18, we have assessed the radiation tolerance of CeBr 3 scintillators to solar proton events (SPEs). Again, 4 samples of dimension of 1 in. 1 in. were irradiated with increasing proton fluences starting at 10 9 protons/cm 2 and then 10 10, 10 11, and protons/cm 2 and with the proton energies replicating the slope of the August 1972 SPE energy spectrum. 19 As for LaBr 3 (Ce 3+ ), the irradiations were performed with the AGOR superconducting cyclotron at the Kernfysisch Versneller Instituut (KVI) located at Groningen, The Netherlands. 20 As already reported in Ref. 4, CeBr 3 energy resolution and light yield are barely affected even by the highest fluence of protons/cm 2, which corresponds to 1 Mrad equivalent dose in Si. After such a dose, the measured FWHM energy resolution at 662 kev was 4.6%, which compares favorably to the pre-irradiation value of 4.5%. A similar tolerance to radiation was observed with LaBr 3 (Ce). 18 In Fig. 7, we compare proton activation spectra of CeBr 3 and LaBr 3 (Ce 3+ ) after irradiation with the highest fluence FIG. 7. Comparison of intrinsic activation spectra acquired 18 days after irradiation with protons/cm 2 of both CeBr 3 and LaBr 3 (Ce 3+ ) encapsulated samples. LaBr 3 (Ce 3+ ) data were collected during previous irradiation campaigns. Apart from an excess in activity in the CeBr 3 spectra between 150 kev and 200 kev, the two activation spectra are essentially the same. C. Minimum detection limits to gamma-ray line and continuum sources The background environment at Mercury will be the same for both spectrometers. We have estimated the level of the instrumental background by scaling the measured background of the MESSENGER GRS 21 which is composed of two principal components a continuum component arising from gamma-ray emission from the planet and a local component due to the cosmic rays and their secondaries generated within the material of the instrument and spacecraft itself. The expected planetary gamma-ray flux detected by the MGNS was estimated from the difference in low-altitude (below 2000 km) and high-altitude (above 8000 km) MESSENGER spectra, normalized by ratio of the projected detector areas and relative efficiencies. The local production component of MGNS was estimated by scaling the MESSENGER high-altitude spectra by the ratio of detector volumes. Using the estimated background fluxes, the detection limits of both the FM and FSM spectrometers to un-broadened line and continuum fluxes has been evaluated following the treatment of Jacobson et al. 22 who showed that the sensitivity of a gamma-spectrometer to detect gamma-ray fluxes in the presence of a background is given by F m = k 2εAG α dω 1 ( ) ( ) k 2 ( + k 1 T + 4R b E + 1 ) 2, (3) s T s T s T b where F m is the minimum detectable source flux in units of photons cm 2 s 1 for lines or photons cm 2 s 1 kev 1, for continuum, k is the number of sigma the signal is detected over the background, ε is the detector efficiency, A is the detection area in cm 2, α is a transmission factor accounting for material between the source and detector, dω is the solid angle, integrated over the source, T s is the time spent observing the source, T b is the time spent acquiring the background, R b is the detector background in counts s 1 kev 1, E is the considered energy bandwidth, and G is a function equal to E for continuum fluxes and 1.2 times the FWHM energy resolution for line fluxes. For the purpose of comparison, we have assumed a point source model and applied a simple correction for solid angle subtended to the source. The

8 Kozyrev et al. Rev. Sci. Instrum. 87, (2016) to the poorer efficiency (as an example, we note that the full energy peak efficiency at 3 MeV is 6% for a 5 cm HPGe crystal, whereas it is 30% for a 3-in. CeBr3 crystal). For the detection limits of continuum, we have assumed a bandwidth of 0.1E. From Fig. 8, we see that CeBr3 is now better than LaBr3(Ce3+) up 3 MeV and the same above this energy. As expected, both detectors have a much better detection limit than HPGe across the entire energy range, since the resolution function now has no effect. V. CONCLUSIONS FIG. 8. The calculated minimum detection limits to un-broadened lines and continuum fluxes of both spectrometers for a s on-source observation, a s off-source observation and a significance of 3 sigma in the detected flux (see also Table I). For the continuum detection limits, we have assumed a bandwidth of 0.1E. For comparison, we also show the calculated detection limits for a 5 cm diameter 5 cm thick HPGe detector, representative of that flown on MESSENGER.23 results, shown in Fig. 8 and presented in Table I, correspond to an exposure time of s (i.e., s observing the source (planet) and s for determining background) and a significance of 3 sigma in the detected flux. It can be seen that the minimum detection limits of CeBr3 are lower than LaBr3(Ce3+) over the critical 500 kev 3 MeV range where most of the lines generated by the planet are expected. Above 3 MeV, the background due to 138La is now effectively zero and so the LaBr3(Ce3+) detection limits are now lower than CeBr3, purely due to the improved energy resolution. For CeBr3, the minimum detection limits to un-broadened lines were found to be photons cm 2 s 1. For comparison we also show the detection limits of a 5 cm diameter 5 cm thick HPGe detector, representative of that flown on MESSENGER.23 We can see that in the few hundred kev region, the detection limits of lines are better than for CeBr3 due to the better energy resolution, but are considerably worse in the MeV region due FIG. 9. The MGNS instrument integrated into the BepiColombo Mercury Polar Orbiter flight spacecraft. The black cylinder on the right hand side contains the CeBr3 crystal and its PMT. In this orientation, the planet is downwards. The results of these measurements presented here support the decision to replace the original LaBr3(Ce3+) crystal as the primary gamma-ray sensing element of the MGNS with an equivalently sized CeBr3 crystal and will increase the science return of the MGNS both in terms of the quality and the quantity of scientific data. The MGNS flight spare model was delivered to the BepiColombo project and has been integrated into MPO spacecraft (see Fig. 9). ACKNOWLEDGMENTS The authors wish to thank Larry Evans of the MESSENGER Science Team for providing MESSENGER data and the BepiColombo project team for technical support. The work in Russia was supported by Grant RNF No of the Russian Science Foundation. The development and commissioning of the transportable gamma-ray station in Czech Republic was funded by Research Grant No. AO/16647/10/NL/CBi of the European Space Agency. Storage and operation of the station in the Czech Republic are supported by Grant research infrastructure No. LM / M of the Ministry of Education, Youth, and Sports of the Czech Republic. 1E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. Krämer, and H. U. Güdel, High-energy resolution scintillator: Ce3+ activated LaBr3, Appl. Phys. Lett. 79, 1573 (2001). 2A. Owens, Scintillators on interplanetary space missions, IEEE Trans. Nucl. Sci. 55(3), 1430 (2008). 3I. G. Mitrofanov et al., Mercury gamma and neutron spectrometer MGNS for planetary orbiter of the BepiColombo mission, Planet. Space Sci. 58(1-2), 116 (2010). 4F. G. A. Quarati, P. Dorenbos, J. van der Biezen, A. Owens, M. Selle, L. Parthier, and P. Schotanus, Scintillation and detection characteristics of high-sensitivity CeBr3 gamma-ray spectrometers, Nucl. Instrum. Methods Phys. Res., Sect. A 729, 596 (2013). 5R. Schulz and J. Benkhoff, BepiColombo: Payload and mission updates, Adv. Space Res. 38, 572 (2006). 6T. Harrington, J. Marshall, J. R. Arnold, L. Peterson, J. Trombka, and A. Metzger, The Apollo gamma-ray spectrometer, Nucl. Instrum. Methods 118, 401 (1974). 7W. Boynton et al., Expected Performance and Initial Results From the 2001 Mars Odyssey Gamma Ray Spectrometer (GRS) Instrument Suite, Abstract No (EGS XXVII General Assembly, Nice, April 2002). 8J. Trombka et al., Remote planetary geochemical exploration with the NEAR X-ray/gamma-ray spectrometer, Nucl. Instrum. Methods Phys. Res., Sect. A 422(1-3), 572 (1999). 9J. Bruckner and J. Mazarnik, Planetary gamma-ray spectroscopy of the surface of Mercury, Planet. Space Sci. 45, 39 (1997). 10W. V. Boynton et al., The Mars Odyssey gamma-ray spectrometer instrument suite, Space Sci. Rev. 110(1), 37 (2004). 11R. S. Saunders et al., Mars Odyssey mission summary, Space Sci. Rev. 110(1-2), 1 (2004).

9 Kozyrev et al. Rev. Sci. Instrum. 87, (2016) 12 C. Granja et al., A transportable source of gamma-rays with discrete energies and wide range for calibration and on-site testing of gammaray detectors, Nucl. Instrum. Methods Phys. Res., Sect. A 771, 1 (2015). 13 Yu. A. Alexandrov et al., in Elementary Particles, Nuclei and Atoms: Low Energy Neutron Physics, Part 1, edited by H. Schopper (Springer-Verlag, Berlin, Heidelberg, New York, 2000), ISBN: D. B. Pelowitz et al., MCNPX 2.7.E Extensions, LA-UR (Los Alamos National Laboratory, 2011). 15 A. Owens, Spectral degradation effects in an 86 cm 3 coaxial Ge(HP) detector, Nucl. Instrum. Methods Phys. Res., Sect. A 238, 473 (1985). 16 D. Smart and M. Shea, A review of solar proton events during the 22nd solar cycle, Adv. Space Res. 30, 1033 (2002). 17 S. F. Mughabghab, Neutron Cross Sections: Neutron Resonance Parameters and Thermal Cross Sections. Part A, Z = 1 60, 4th ed. (Academic Press, New York, 1981). 18 A. Owens et al., Assessment of the radiation tolerance of LaBr 3 :Ce scintillators to solar proton events, Nucl. Instrum. Methods Phys. Res., Sect. A 572, 785 (2007). 19 A. Owens et al., An assessment of radiation damage in space-based germanium detectors due to solar proton events, Nucl. Instrum. Methods Phys. Res., Sect. A 583, 285 (2007). 20 See for Kernfysisch Versneller Instituut (KVI), University of Groningen. 21 L. G. Evans et al., Major-element abundances on the surface of Mercury: Results from the MESSENGER gamma-ray spectrometer, J. Geophys. Res.: Planets 117, E00L07, doi: /2012je (2012). 22 A. S. Jacobson, R. J. Bishop, G. W. Culp, L. Jung, W. A. Mahoney, and J. B. Willet, A balloon-borne Ge(Li) spectrometer for gamma-ray astronomy, Nucl. Instrum. Methods 127, 115 (1975). 23 J. O. Goldsten et al., The MESSENGER gamma-ray and neutron spectrometer, Space Sci. Rev. 131(1), 339 (2007).

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