A transportable source of gamma rays with discrete energies and widerange for calibration and on-site testing of gamma-ray detectors

Carlos Granja a,n, Tomas Slavicek a, Martin Kroupa a,1, Alan Owens b, Stanislav Pospisil a,Zdenek Janout a, Miloslav Kralik c, Jaroslav Solc c, Ondrej Valach a

a Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, 12800 Prague 2, Czech Republicb European Space Technology Centre ESTEC, European Space Agency ESA, Keplerlaan 1, 2200AG Noordwijk, The Netherlandsc Czech Metrology Institute, Radiova 3, 102 00 Prague 10, Czech Republic

a r t i c l e i n f o

Article history:Received 20 August 2014Received in revised form30 September 2014Accepted 1 October 2014Available online 28 October 2014

Keywords:Transportable gamma-ray sourceGamma-ray detector calibrationMonte Carlo simulationDose rate measurementSpacecraft payload qualification

a b s t r a c t

We describe a compact and transportable wide energy range, gamma-ray station for the calibration ofgamma-ray sensitive devices. The station was specifically designed for the on-site testing and calibrationof gamma-ray sensitive spacecraft payloads, intended for space flight on the BepiColombo and SoIarOrbiter missions of the European Space Agency. The source is intended to serve as a calibrated referencefor post test center qualification of integrated payload instruments and for preflight evaluation ofscientific radiation sensors. Discrete gamma rays in the energy range 100 keV–9 MeV are produced inthe station with reasonable intensity using a radionuclide neutron source and 100 l of distilled waterwith 22 kg salt dissolved. The gamma-rays generated contain many discrete lines conveniently evenlydistributed over the entire energy range. The neutron and gamma-ray fields have been simulated byMonte Carlo calculations. Results of the numerical calculations are given in the form of neutron andgamma-ray spectra as well as dose equivalent rate. The dose rate was also determined directly bydedicated dosemetric measurements. The gamma-ray field produced in the station was characterizedusing a conventional HPGe detector. The application of the station is demonstrated by measurementstaken with a flight-qualified LaBr3:Ce scintillation detector. Gamma-ray spectra acquired by bothdetectors are presented. The minimum measuring times for calibration of the flight-version detector,was between 2 and 10 min (up to 6.2 MeV) and 20–30 min (up to 8 MeV), when the detector was placedat a distance 2–5 m from the station.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Radiation measurement instruments such as gamma-ray detec-tors used for space and planetary missions [1] generally requireradiation sources of discrete energy and wide-dynamic range [2,3]for testing and response calibration. However, devices alreadyintegrated and installed on spacecraft platforms would welcome atransportable source of discrete (mono-energetic) gamma rayscovering a wide energy range especially for pre-flight verification.The required tasks include characterization, optimization andcalibration at the high-energy range of gamma-ray detectors suchas scintillating [1,4,5] and position-sensitive semiconductor [6]devices. For these purposes, a small, transportable gamma-raysource that provides discrete peaks over a wide-energy range(100 keV–9 MeV) has been comstructed. The station consists of a

compact (o2 cm) radioactive neutron source of limited activity(�Ci) and a container (o60 cm) of neutron moderating andgamma-ray converter material such as water and salt, which canbe prepared and loaded on site.

The newly-built station is a compact and transportable versionof a fully configurable in-house stationary gamma-ray station [2].The stationary station is based on a radioactive neutron source(AmBe or 252Cf) and stackable moderating/converter segments ofmaterial/elements of varying volume and content. The stationprovides gamma rays over a energy and intensity range beadjusted the configuration of the station. Other sources such asaccelerators provide few peaks in a limited energy range incomparison with a neutron generator based source [3]. Theoptional benefit of enhanced suppression of low-energy gammarays comes at the price of providing just one gamma-ray energy oflimited intensity [3].

We present the design, evaluation and operation of thetransportable station together with measurement of the gamma-ray field obtained with gamma-ray detectors. The chosen moder-ating and gamma-ray converting material assembly consists of a

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journal homepage: www.elsevier.com/locate/nima

Nuclear Instruments and Methods inPhysics Research A

http://dx.doi.org/10.1016/j.nima.2014.10.0010168-9002/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author . Tel.: þ420 224 359 394; fax: þ420 224 359 392.E-mail address: carlos.granja@utef.cvut.cz (C. Granja).1 Presently at University of Houston, 4800 Calhoun Rd., 77004 Houston, TX,

USA.

Nuclear Instruments and Methods in Physics Research A 771 (2015) 1–9

compact AmBe neutron source of activity 1 Ci coupled to asolution of standard salt and distillated water. The later materialcan be filled on site. Provisions are required for the radionuclidetransport, delivery, handling and storage of the radioactive sourceat the test centers. Monte Carlo simulations were carried out toverify and optimize the spectrum and production yield of thegamma field across the station volume. For radiation dosimetry ofthe measured devices, as well as operator radiation protection,additional numerical simulations were performed to determinethe dose rate and dose equivalent of the neutron and gammabackground fields at the measuring and operation positions on thesurface and around the station. Measurements of the gamma-rayfield were performed with a reference HPGe gamma-ray detector.Testing and calibration of was also carried out using a LaBr3:Cescintillating gamma-ray detector [7] provided by the EuropeanSpace Agency (ESA).

2. The transportable gamma-ray station

2.1. The production of discrete gamma rays in wide range with aradioactive neutron source

Prompt gamma rays are generated by neutrons following twoprocesses [8–10]: radiative capture of thermal neutrons—i.e. (n,γ)reaction, and inelastic scattering of fast neutrons—i.e. (n,n',γ)reaction. High-energy gamma rays can be produced only byradiative neutron capture which requires moderation of neutronsto thermal energies. Neutrons are most readily thermalized bylow-Z materials such as water, heavy water, graphite and paraffin.The energies of the resulting gamma rays correspond to discretetransitions between well-defined energy states of the residualtarget nuclei; hence they are discrete, characteristic and unique foreach material.

2.2. Station design, radiation source, moderation and convertermaterial

The transportable station is based on the design of the stationarygamma-ray facility [2]. It consists of a compact radionuclideneutron source and a container of light material to house themoderator and converter material (Fig. 1). The neutron source usedis a closed source AmBe (type Am1.N09) of activity 3.7�1010 Bq,size 17.4 mm�19.2 mm and neutron emission rate 2.1�106 s�1.The main container is filled with material containing moderatingelements—such as hydrogen, carbon and oxygen. Hydrogen isindeed the most efficient moderator. We designed, modeled andtested different configurations of the assembly (composition andrelative content) taking into account their moderating efficiency,

gamma-ray production yield, spectrum (energies, intensities) andrange of gamma rays. We concluded that a a simple configuration of100 l of distillated water (H2O) and conventional (edible) salt(sodium chloride)—22 kg diluted was optimal for general calibra-tion work. Additional biological shielding of lithium-doped poly-ethylene (PE) can be attached for radiation protection of operationpersonnel when required. The whole loaded assembly weighs lessthan 170 kg.

The station is assembled of several functional parts (see Fig. 1).The main component is the square shaped hollow container (ofdimensions 50 cm�56 cm�56 cm) made of polymethyl metha-crylate (PMMA) plastic (see Fig.1(c)). The container is equippedwith a flow inlet and outlet for loading the moderating liquid(water) as well as an expansion chamber for water volumefluctuations including possible temperature variations. On thetopside of the container a cylindrical central hole accommodatesa cork element, which is constructed as a separate component toload and house the radioactive neutron source (see Fig. 1(d)). Thecork is made of the same material (PMMA) and is filled also withthe same solution as the main container. Sealed flow inlet andoutlet assure clean operation of the station.

2.3. Loading and operation of the station

The loading and operation of the station proceeds in thefollowing several steps (illustrated in Fig. 2):

� Retrieval of the radioactive AmBe source (from the storage andshielding transport container) – Fig. 2a.

� Positioning the AmBe source in the neutron source holder. Forthis purpose a source handling plate is provided which isequipped with a source loading and releasing benches (shownon left and right, respectively, in Fig. 2(b)). The source isinserted into the bore opening of the holder (Fig. 2(b)). Theholder has pins on the sides, which fit into a locking clip. Thebottom element of the holder is equipped with locks and pinsthat fix the holder preventing rotation. The bench on the right(red lid) is made for the release of the neutron source from theholder (during source unloading). The source manipulationplate, used for fixed insertion and removal of the neutronsource into and from the cork unit, can be stored underneaththe transportation cart (see below, see also Fig. 3(d)).

� The holder loaded with the neutron source is then placed insidethe cork element, which is held and handled by an aluminumbar (Fig. 2(c)). The cork is filled with the same solution as themain container. The element is equipped with fastening latchthat guarantees that the neutron source holder is not releasedduring manipulation. The cork is equipped with clips formanipulation of the detachable holder with the radioactive

Fig. 1. Illustration (a) of the transportable gamma-ray station (b). The container (c) is equipped with an expansion chamber and flow inlet and outlet as well as the centralcork (d) containing the neutron source. The overall size and main parts are indicated. Labels for two measuring positions (α, β) are included (b).

C. Granja et al. / Nuclear Instruments and Methods in Physics Research A 771 (2015) 1–92

source. Loading, handling and unloading of the AmBe source iskept at safe distance (42 m). The cork unit is also used fortransportation of the neutron source between the experimental/measuring area and the source storage depository (Fig. 2(a)).

� The loaded cork is transported using a transversal bar securedthrough the clips attached (Fig. 2(d)). Manipulation and mobilityof the neutron source is thus kept at a safe distance (� 2 m).

� The loaded cork with the neutron source is deposited into thestation main container, as shown in Fig. 2(e). (the stationarygamma-ray station [2] as well as the detector signal electronics(NIM modules and NIM crate) appear in the background). Thecontainer is filled beforehand with the moderating/convertersolution of distilled water and NaCl. The whole station istransported on a cart.

Fig. 2. Loading of the station—steps (see text) (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig. 3. Unloading of the neutron source—steps (see text).

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� Filling of the station main container via the flow inlet tight valve(Fig. 2(f)). Both the cork unit and the main container are equippedwith expansion chambers to avoid spills or water leaks.

� In photograph Fig. 2(g) we show the transportable stationdeployed on the movable cart. The station is shown onlypartially filled with the moderating/converter solution beforecomplete installation of the expansion chambers and priorloading of the neutron source.

� Fig. 2(h) shows details of the station bottom corner includingthe drain valve used as a flow outlet to empty the maincontainer.

2.4. Unloading the neutron source

The unloading of the AmBe source follows several steps(illustrated in Fig. 3)

� After the measurement, the cork unit is removed from thesource by lifting it with the transversal bar and the cork'sholding clips. The cork unit is then placed (standing vertically)on the source handling plate (Fig. 3(a)). By rotation over thehorizontal plane, the source holder cylinder is released fromthe cork.

� The source holder cylinder is transferred from the sourceloading bench (shown left in Fig. 3(b)) to the source releasebench (on the right in Fig. 3(b)) in order to release the neutronsource from the holder cylinder.

� The neutron source is then released from the holder cylinder bymeans of a vertical pin passing through the holder cylinderfrom the bottom (Fig. 3(c). The neutron source is then displacedto the upper position of the holder cylinder where it can beheld by pliers (Fig. 3(c)) to be removed and stored back into thetransport barrel (Fig. 2(a)).

� The neutron-handling bench can be stored underneath thestation cart (Fig. 3(d)).

2.5. Radiation safety

Loading and unloading of the station as well as handling andoperation of the cork unit with the neutron source, has to becarried out by trained personnel in radiation protection, in linewith national regulations of radiation safety for handling of theneutron radiation source. Levels of dose equivalent rate for theloaded station under operation conditions were simulated (seeSection 2.9) and also measured (see Section 2.10).

2.6. Transportability and mobility

The source is specifically designed to be transportable toremote and on-site places such as laboratories abroad and havemobility between experimental areas. The loaded station can bemoved by cart (Fig. 4(a)) and also lifted by crane to reach thedesired position around the detector assumed to be mounted on aspacecraft. The radioactive source may be either provided on siteor it can be transported, separately, by a licensed carrier. Thecontainer shown in Fig. 2(a), is used for the transport and shippingof the radioactive source.

2.7. Loaded cork unit as optional detachable gamma-ray source

The cork element loaded with the radioactive source and filledwith moderating/converter material can also serve as a simplifiedgamma-ray source itself (Fig. 4(b)). Detached from the maincontainer, it can be handled and transported by the holding clips

equipped on the top side of the cork container. This compacthand-held unit has reduced moderating power and is intended forshort-term stimulation of devices.

2.8. Monte Carlo simulations of the neutron and gamma-ray field

The neutron and gamma fields in the station were simulatedusing the MCPX5 Monte Carlo Code [11,12]. Results are presentedfor the loaded station with the chosen configuration (1 Ci AmBesource, 22 kg salt and 100 l of distillated water).

The distributions of neutron flux across the station centerthrough the location of the neutron source are shown along thehorizontal and vertical planes in Figs. 5 and6, respectively. Resultsare given for all neutrons as well as for the thermal and fastcomponents. Values given, normalized to one emitted neutron,can be multiplied by the source emission (2.1�106 neutrons/s) togive absolute flux values.

The spatial distribution of gamma-ray flux across the stationcenter through the position of the neutron source is shown along thehorizontal and vertical planes in Fig. 7. The given values correspondto the integrated gamma-ray energy spectrum (�50 keV to 9 MeV).

The neutron spectra simulated at two selected positions out-side the station are shown in Fig. 8. The positions correspond tothe horizontal plane crossing the neutron source (along thedirection of position labels α–β shown in Fig. 1(b)) at 2 m and5 m from the station center (i.e. at the position of the neutronsource as loaded). The neutron spectra exhibit maxima at around2–3 MeV and in the thermal range, a broad, partly-thermalizedfield. Between the measured positions the flux decreases aboutone order of magnitude along the entire range.

The gamma-ray spectra simulated at the same positions as forthe neutron spectra above are given in Fig. 9. The discrete peakstructure is resolved above the broad continuum. Between themeasured positions, the flux decreases about one order of magni-tude along the entire range.

2.9. Dose rate: simulation

The dose equivalent rate was also calculated by Monte Carlosimulations. Calculations were carried out for the loaded stationand in the chosen configuration (Section 2.2) at the selectedpositions (Section 2.8) outside of the station main container (at2 m and 5 m over from the neutron source along the directionindicated in Fig. 1(b)). Results are listed in Table 1 in units of mSv/h.Uncertainties are less than 5% (statistical uncertainty is 0.5%,uncertainties in (n,γ) cross-section is 4%).

2.10. Dose rate: measurement

The ambient dose equivalent rate for neutrons and photonswas measured for the loaded station in the loaded configuration(Section 2.2). The measurements were made using a neutron surveymeter (Berthold, model LB 6411) and a photon survey meter (RadosTechnology Oy, model RD 200), respectively. Two positions wereselected over the plane of the neutron source along the directionindicated in Fig. 1(b): on the surface (lateral side) of the station. Oneposition is on the station surface by the container wall at 30 cmfrom the neutron source (position α in Fig. 1(b)), and at 2 m(position β in Fig. 1(b)) from the neutron source/station center.The results are given in Table 2 in units of mSv/h. The uncertainty isat the level 10––20%. The natural background level measured wasmeasured to be 0.3 mSv/h.

The measured values agree with the corresponding Monte Carlovalues (Section 2.9 and Table 1) within 40% including uncertainties.The difference between the measured and calculated neutron doserate is most likely due to the difference in moderation from the

C. Granja et al. / Nuclear Instruments and Methods in Physics Research A 771 (2015) 1–94

surrounding environment where a simplified model was used for thenumerical calculation.

3. Gamma ray measurements with HPGe and LaBr3:Cedetectors

The experimental verification of the gamma-ray field provided(Section 2.2) was performed using a high efficiency HPGe gamma-ray detector. The detector (ORTEC, model GMX35-76-CW-PL),containing a crystal of size of 54.9 mm diameter and 81.2 mmlength, was readout by an embedded charge sensitive preampli-fier. The detector was operated at a bias of �3 kV. Measurementswere carried out using a pulse-shaping time of 6 ms and anamplifier gain of �10. A signal amplifier (ORTEC, model 673)and multichannel analyzer (ORTEC, model MCA ASPEC 927)completed the signal chain.

The station was validated by testing and characterization oftwo gamma-ray sensitive device developed by ESA for futureremote sensing planetary missions [7]. One device was based onLaBr3:Ce. The other on LaCl3:Ce. The devices tested are listed inTable 3. The detector photo-multiplier tube was operated atþ600 V. The data acquisition was done with the signal amplifierand MCA indicated above.

Both detectors were also used to measure the background field(i.e. without the AmBe source). The acquired gamma-ray spectra,of the loaded and unloaded station, are given below for eachdetector separately.

3.1. HPGe detector

The gamma-ray field was measured without the neutronsource loaded (background measurement) and with the stationloaded in the chosen configuration (i.e. 100 l distilled water, 22 kg

Fig. 4. (a) Fully-loaded station (neutron source inside cork, container tank filled). (b) Cork element with loaded AmBe neutron source serving as highly-compact, hand-heldgamma-ray source. (c) Tested scintillating LaBr3:Ce detector (two radioactive sources used for efficiency for calibration are visible—on the top right side).

Fig. 5. Spatial distribution of the simulated neutron flux over the horizontal plane crossing the AmBe source (at position x¼0 cm, y¼0 cm): all neutrons (a), fast neutrons(b), thermal neutrons (c). The neutron flux (#/cm2) is given relative per one neutron emitted from the AmBe source.

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salt). The gamma-ray spectra acquired are given in Fig. 10. Theresponse over the entire energy range (displayed on the top plot)is also shown in consecutive energy range segments.

The spectra were calibrated for energy using well-knows linesfrom 35Cl, 60Co, 137Cs, and 12C [13–15]. The efficiency calibrationwas achieved with calibrated gamma-ray emitter standards 60Co,137Cs, 152Eu of defined absolute activity. The calibrating sourceswere measured at two positions: 10 cm and 2 m from the detector.The resulting values of detector efficiency range from about 50% at150 keV to about 10% at 1.5 MeV.

The background spectrum is characterized by gamma-ray linesbelow 2.7 MeV namely from 208Tl, 208,212,214Bi, 228Ac, 40K, 212,214Pb,

224Ra. The largest peaks are 1460.8 keV (40K) and 2614.5 keV(208Tl). Compared to the measured spectrum with the loadedstation, the background continuum is 2–3 orders of magnitudelower in the low-energy region (o3 MeV) and around 3 orders ofmagnitude lower in the high-energy region (43 MeV).

The spectrum measured with the loaded station is characterizedby a fair number of well-established gamma-ray lines mainly from35Cl, which are evenly distributed over the entire energy range (from786.3 keV up to 8578.6 keV). For many high-energy peaks in thespectrum, in addition to the full-energy peaks also single- (SE) anddouble-escape (DE) peaks are registered and resolved. Other gamma-ray lines registered, mostly of lower intensity and fewer in number,

Fig. 6. Same as Fig. 5 for the vertical plane crossing the AmBe source (at position x¼0 cm, z¼25 cm): all neutrons (a), fast neutrons (b), thermal neutrons (c). The neutronflux (#/cm2) is given relative per one neutron emitted from the AmBe source.

Fig. 7. Monte Carlo simulation of the distribution of gamma-ray flux produced along the horizontal (a) and vertical (b) planes crossing the AmBe source at x¼0 cm, y¼0 cmand z¼25 cm. The gamma-ray flux (#/cm2) is given relative per one neutron emitted from the AmBe source.

C. Granja et al. / Nuclear Instruments and Methods in Physics Research A 771 (2015) 1–96

correspond to 56Fe, 28Si, 48K, 56Fe and 1H. The broadened peakstructures at 4438 keV, 3927 keV and 3416 keV correspond to full-energy, single- and double-escape peaks of neutron inelastic scatter-ing in 12C. The annihilation peak at 511 keV is also clearly resolved.

3.2. LaBr3:Ce detector

The gamma-ray field produced by the loaded stationwas measuredwith the LaBr3:Ce detector listed in Table 3 (see also Fig. 4(c)). Inanalogy to the measurements with the HPGe detector, gamma-rayspectra were collected with the neutron source unloaded (backgroundmeasurement) and with the station loaded in the chosen configuration

(i.e. 100 l distilled water, 22 kg salt). The spectra for the loaded (at 2 m)and unloaded station each acquired in 120min are given in Fig. 11.

The spectrummeasured with the loaded station is characterized bythe most intense gamma-ray lines from 35Cl evenly distributed overthe entire energy range (from 1164.9 keV up to 8578.6 keV). Otherlarge gamma-ray peaks resolved correspond to 208Tl, 138La, 214Bi and1H. Most high-energy peaks in the spectrum correspond to fullabsorption peaks with a few peaks registered corresponding to single-and double-escape peaks. Other gamma-ray lines registered, mostly oflower intensity and few in number, correspond to 56Fe, 28Si, 40,48K, 56Feand 1H. Broaden peak structures corresponding to neutron inelasticscattering in 12C (present in the source and station material) at4438 keV and 3927 keV are resolved—corresponding to full energyand single-escape peaks. The 511 keV annihilation gamma-ray peak isalso clearly registered. The large, partly-broadened peak at 1436 keV(138La)þ1460 keV (40K) and the broad structure at 750–950 keV (alloriginate in the detector itself as internal radiation of radioactivedopants) are dominant peaks in all spectra (background, and loadedstation). Close-lying lines appear as broad peaks e.g. at around1955 keV and 6610 keV which correspond to 3-line multiplets:2975.2 keV (35Cl DE)þ1951.1 keV (35Cl)þ1959.4 keV (35Cl) and6111 keV (35Cl)þ6620 keV (35Cl SE)þ6628 keV (35Cl SE), respectively.

Spectra were collected at two positions (at 2 m and 5m from thestation center/neutron source) and at varying total acquisition time(1 min, 2 min, 3 min, 4 min, 5 min, 10 min, 20 min, 30 min, 60 minand 120min). The large doublet peak at 1436 keV, 1460 keV and thebroad bump at 750–950 keV (internal radiation in the detector) areresolved in a fraction of a second (o0.2 s) at both positions. The peaksat 1951 keVþ1959 keV (both 35Cl), 2223 keV (1H) and 2614 keV (208Tl)are resolved (i.e. the peak is clearly visible and can be unambiguouslyfitted) in 1 min and 2min at 2 m and 5m, respectively. The carboninelastic scattering peak structures at 4438 keV and 3927 keV areresolved in 1 min and 2min at 2 m and 5m, respectively. The high-energy composite peak at 6110 keVþ6619 keV (single escape)þ6628 keV (single escape) all from 35Cl are resolved in 2 min and10min at 2 m and 5m, respectively. The higher-energy peaks at6619 keVþ6628 keV, 7414 keV and 7790 keV (all 35Cl) are resolved in20 min and 30min at 2 m and 5m, respectively. The highest energy

Fig. 8. Simulated neutron spectra at 2 m (blue) and 5 m (red) from the center of the station along the direction shown in Fig. 1(b) (labels α–β). Results are given for theloaded station equipped with the AmBe source used (emission 2.1�106 n/s) (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)

Fig. 9. Same as Fig. 8 for the gamma-ray spectra calculated at the same positions: 2 m (blue), and 5 m (red) (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)

Table 1Monte Carlo simulation of dose equivalent rate H(10) for the loaded station equippedwith an AmBe source used (emission 2.1�106 n/s). Values are given for the gamma andneutron field components separately at distance 2 m and 5m from the station center(neutron source position) along the direction indicated in Fig. 1(b). Uncertainties arearound 5%.

Radiation Quantity Hn(10) Hn(10)

Position¼ 2 m 5 m

Neutrons 1.06 mSv/h 0.15 mSv/hPhotons 0.19 mSv/h 0.03 mSv/h

Table 2Experimental measurement of the dose rate for the loaded station on the surfaceand at 2 m from the station center (neutron source position) along the directionindicated in Fig. 1(b). Results are given for the gamma and neutron fieldcomponents. Uncertainties are at the level 10–20%.

Radiation Quantity Hn(10) Hn(10)

Position¼ Station surface 2 m

Neutrons 31 mSv/h 2.20 mSv/hPhotons 40 mSv/h 0.62 mSv/h

C. Granja et al. / Nuclear Instruments and Methods in Physics Research A 771 (2015) 1–9 7

peak at 8578 keV (35Cl) is resolved in 60 min and 120min at 2 m and5m, respectively.

From these measurements can be concluded that the requiredminimum time periods for complete calibration of the LaBr3:Cedetectors are 1–2 min (up to 4.5 MeV), 2–10 min (up to 6.2 MeV),20–30 min (up to 8 MeV) and 1–2 h (up to 8.6 MeV) at 2 m and5 m, respectively.

4. Conclusions

The station is designed for instrument qualification and preflightcalibration of space radiation sensors on site such as at the testcenters of space agencies or the large scale integrators. Thetransportable system enables the wide energy range testing ofremote sensing gamma-ray detection systems. Gamma rays are

Fig. 10. Gamma-ray spectra measured with the HPGe detector with the station loaded (blue curve). The detector was placed at 2 m from the station center. Background datafor the unloaded station without the neutron source is included (black curve). The whole spectrum (top) is shown for detail in consecutive limited energy regions (second tosixth plots from top). Data were collected during 17 h 48 min for the loaded and 31 days for the unloaded station. The data for the background measurement displayed isnormalized to the time of the loaded data. Energies for several peaks are indicated (values rounded to keV) including labels for single-escape (SE) and double-escape (DE)peaks (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 3Details for gamma-ray scintillation detectors used in this work. The model reference and crystal dimensions are given.

Detector Crystal size

LaBr3:Ce (BriLanCe 380)–ESA k673 (flight version) 76 mm�76 mmLaCl3:Ce (BriLanCe 350)–ESA 102Yl152 102 mm�152 mm (4''�6'')

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obtained from thermal neutron capture, which is a convenient andcost efficient way of generating discrete gamma rays with energiesabove 3 MeV. The neutron moderating / gamma-ray convertermaterial (a solution of salt in distillated water) can be filled on site.The response of the system was evaluated with a high-efficiency(HPGe) reference detector. The systemwas tested and demonstratedusing an in-flight payload device equipped with a scintillatingLaBr3:Ce detector. The density of calibrating peaks in both types ofdetectors proved reasonably high and evenly spread over the entirerange from �100 keV up to 8.6 MeV. The time required to calibratea device up to 8 MeV is reasonable—no more than 30 min at 5 mdistance from the station. The low-energy background level is keptat reasonable level allowing clear calibrating peaks to rise in lessthan 1 min (below 1 MeV) and 1–2 min (up to 3 MeV) at 5 mdistance. The transportability of the entire system, simple loadingand minimal operation make it attractive for on-site use at remotefacilities and laboratories with only partial restrictions on handlingand transport of the radioactive source. Ambient dose rate duringloaded station operation is reasonably low. The dose rate at 5 m isnearly negligible and at 2 m is only few factors above the naturalbackground level. On the surface of the loaded station including theneutron source the dose rate is only one order of magnitude higherthan the natural background level. The time required for loadingand unloading the neutron source is only few minutes.

Acknowledgments

Construction of the transportable gamma-ray station wasfunded through Research Grant no. AO/1-6647/10/NL/CBi by the

European Space Agency. Storage and operation of the station andAmBe neutron source are supported by Grant research infrastruc-ture no. LM2011030/149-120006M of the Ministry of Education,Youth and Sports of the Czech Republic.

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Fig. 11. Same as Fig. 10 for the LaBr3:Ce detector (flight version). Data are shown for the station loaded (blue curve) with the detector placed at 2 m from the station center.Background data for the unloaded station without the neutron source is included (black curve). Data were collected in 2 h for each measurement. The whole spectrum (top)is shown in two expanded energy regions (middle and bottom). Energies for several peaks are indicated (values rounded to keV) including labels for single-escape (SE) anddouble-escape (DE) peaks. Multiplets (see “3� ” labels) are described in the article text (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

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