A transportable source of gamma rays with discrete energies and wide range for calibration and on-site testing of gamma-ray detectors
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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 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 (, ) are included (b).
C. Granja et al. / Nuclear Instruments and Methods in Physics Research A 771 (2015) 192
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  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 stationsteps (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 sourcesteps (see text).
C. Granja et al. / Nuclear Instruments and Methods in Physics Research A 771 (2015) 19 3
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 radioac...