a transportable source of gamma rays with discrete energies and wide range for calibration and...
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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 keV9 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 2030 min (up to 8 MeV), when the detector was placedat a distance 25 m from the station.
& 2014 Elsevier B.V. All rights reserved.
Radiation measurement instruments such as gamma-ray detec-tors used for space and planetary missions  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 devices. For these purposes, a small, transportable gamma-raysource that provides discrete peaks over a wide-energy range(100 keV9 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 .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 . Theoptional benefit of enhanced suppression of low-energy gammarays comes at the price of providing just one gamma-ray energy oflimited intensity .
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.: 420 224 359 394; fax: 420 224 359 392.E-mail address: email@example.com (C. Granja).1 Presently at University of Houston, 4800 Calhoun Rd., 77004 Houston, TX,
Nuclear Instruments and Methods in Physics Research A 771 (2015) 19
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  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 : radiative capture of thermal neutronsi.e. (n,)reaction, and inelastic scattering of fast neutronsi.e. (n,n',)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 . 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 3.71010 Bq,size 17.4 mm19.2 mm and neutron emission rate 2.1106 s1.The main container is filled with material containing moderatingelementssuch 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 cm56 cm56 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 benc