Development of low noise scintillator crystals for planetary space missions

C.C.T Hansson, A. Owens, B. Shortt , P. Dorenbos, F. Quarati, R. Williams, D. Hahn, T. Toepfer, L. Pathier, P. Schotanus, J. v.d. Biezen, K.N. O'Neill, C. Jackson, L.Wall.

Abstract – The recent development of large volume

lanthanum halides (LaBr3 and LaCl3), has made

scintillator based detectors a viable option for use as

high resolution Ȗ-ray spectrometers for a large number

of space applications. The main drawback of both LaCl3

and LaBr3 is the high background noise inherent to the

crystal due to the decay of radioactive 138

La, which

comprises 0.09% of naturally occurring stable

lanthanum (139

La). This paper describes the

development of a low noise equivalent to LaBr3, namely

CeBr3. Crystal sizes up to 3” have successfully been

grown. The energy resolution of CeBr3 (4.1% FWHM at

662keV) was found to be slightly worse than that

observed for LaBr3 (2.8% FWHM at 662keV) but a

measured relative decrease of almost 30 times in

internal activity makes it an attractive alternative when

low noise operation is required. In parallel to the CeBr3

development, a S ilicon Photo Multiplier (SPM) aimed at

having a response matched to both LaBr3 and CeBr3

has also been developed. Preliminary results are

presented.

I. INTRODUCTION

Gamma-ray spectrometers have long been used in a number of space borne applications such as planetary surface composition mapping. The basic remote sensing principle which allows for planetary surface composition mapping is illustrated in Figure 1. As solar X-rays impinge on a planet they activate

C.C.T Hansson, A. Owens, B. Shortt and J. v.d. Biezen are with the European Space Agency, Noordwijk, The Netherlands. (conny.hansson@esa.int).

P. Dorenbos and F. Quarati are with the Delft University of Technology, Delft, The Netherlands.

R. Williams is with Praesepe, Groeningen, The Netherlands. D. Hahn and T . Toepfer are with Hellma Materials, GmbH, Jena,

Germany. L. Pathier is with Schott, Mainz, Germany. P. Schotanous is with Scionix, Bunnik, The Netherlands. K. O’Neill, C. Jackson, L.Wall are with SensL Technologies

Ltd, Cork, Ireland.

characteristic X-UD\� IOXRUHVFHQFH�IURP�WKH�WRS����ȝP�of the regolith, while galactic cosmic rays interact with the regolith in the 1-3m depth range producing QHXWURQV�� Ȗ-rays and X-rays. By monitoring the flux and energy spectra of this induced surface radiation the elemental composition of the planet surface can be determined.

Fig. 1. Illustration of the basic principle used for planetary remote sensing in order to map the surface elemental composition.

To date, Ȗ-ray spectrometers used for this application usually require liquid nitrogen cooling to obtain high resolution spectroscopic performance. The need for such cooling sets large demands on limited payload resources and can limit the maximum operational time. To find a suitable candidate material to be used as a non-cooled Ȗ-ray detection media, a number of requirements needs to be considered. First, in order to be able to differentiate between different elemental lines a high energy resolution is needed. Secondly, the flux measured at the spacecraft will be very low, requiring the detector to have a high sensitivity. Until recently, the limited spectral performance of scintillators prevented them from being viable options for use in planetary exploration. This changed with the discovery of the lanthanum bromide (LaBr3) scintillation detector by Delft University of

Technology and the University of Bern [1]. The excellent energy resolution, high sensitivity and room temperature operability made it an ideal candidate for applications in planetary exploration. As such, they were developed into large volume detectors within a European Space Agency (ESA) project and is currently set to fly on ESA’s BepiColombo mission to Mercury [2]-[4]. The main drawback of LaBr3 is its high intrinsic background noise, making crystals unsuitable for use in low noise applications. The underlying cause of this background can be traced back to radioactive 138La, a naturally occurring isotope of La with a 0.0902% abundance and a half-life of 1.05x1011 years [5][6]. This isotope can decay by either electron capture into 138%D� RU� ȕ-decay into138Ce with a 66.4% and 33.5% probability respectively. In either case, the daughter nucleus is created in an excited state and consequently relaxes E\� Ȗ-ray emission (1436keV for 138Ba and 789keV for 138&H��� 7KHVH� Ȗ-rays are then fully or partially absorbed by the LaBr3 crystal causing a high background noise. By substituting the La with Ce, Ce being the dopant used in LaBr3 to create the recombination centres responsible for the scintillation process, a crystal suitable for low noise application while still retaining the positive attributes associated with LaBr3 should be achieved. As such, a low noise scintillator development program is currently being conducted at ESA, looking into the development of CeBr3 (i.e. LaBr3(Ce) with a 100% Ce doping). In this paper the progress of this program up to date is discussed. Currently 2” crystals are routinely produced and 3” crystals have successfully been grown. CeBr3 was found to have an energy resolution that is slightly worse than that observed for LaBr3 but it also shows an almost 30 times reduction in internal activity. The response of CeBr3 and LaBr3 when coupled to a high efficiency blue light sensitive silicon photo-multiplier (SPM), also being developed under a parallel ESA contract, has demonstrated excellent performance.

II. CRYSTAL GROWTH AND PACKAGING

The crystals of CeBr3 were grown by Hellma Materials. Up to date, the increase of available size has been the main focus of the CeBr3 scintillator growth program and 2” crystals, as can be seen in Figure 2 - left, are now routinely produced and 3” crystals have successfully been grown. Due to the hydroscopic nature of CeBr3 the grown crystals are hermetically packaged in Al housings. A quartz window is situated on one end, allowing for the

crystal to be coupled to a standard photo-multiplier tube (PMT) or SPM for detection of the scintillation light. A fully packaged CeBr3 crystal can be seen in Figure 2 – right. The packaging was carried out by Scionix, BV.

Fig. 2. a) Photograph of a 2” CeBr3 crystal. b) Photograph of a fully packaged CeBr3 crystal.

III. CeBr3 SPECTRAL PERFORMANCE

A fully packaged 2” CeBr3 crystal, coupled to a 3” Photonis 5300B PMT, was exposed to a 137Cs radiation source in order to evaluate the spectral performance of the crystal. The response can be seen in Figure 3.

Fig. 3. Spectral response obtained from a 2”CeBr3 crystal when exposed to a 137Cs source.

A spectral resolution of 4.1% full width at half maximum (FWHM) at 662keV was observed. This is slightly worse than that measured for LaBr3 (2.8%

Fig 4. The spectral response of the measured internal activity for LaBr3, CeBr3 and SrI2.

FWHM at 662keV). The cause for the reduction in spectral is currently under investigation but is believed to be due to a higher degree of non-proportionality in CeBr3.

IV. INTERNAL ACTIVITY

Measurements of the internal activity of LaBr3, CeBr3 and SrI2 were conducted. Each crystal was placed in a low activity lead castle in order to minimise any external excitation sources and the internal activity of the crystals themselves was registered using a low 40K content 2” Electrontubes 9266B PMT. The results can be seen in Figure 4. The ����NH9� Ȗ-ray emission peak associated with 138Ba daughter product can clearly be seen in the LaBr3 response. No distinct lines can be observed in the CeBr3 response. A significant reduction in internal activity was seen when comparing CeBr3 to LaBr3 with an internal activity of 0.040 Bq/cm3 and 1.176 Bq/cm3 measured respectively. This constitutes a 30 times reduction in internal activity.

V. RESPONSE OF SCINTILLATOR COUPLED TO SPM

Classical PMT’s are not mechanically robust, suffer from sensitivity to magnetic fields and require high voltage operation. For space applications these factors constitutes significant drawbacks when designing any instrument that is to be flown. SPM’s offer a low voltage, non-vacuum, magnetic field insensitive alternative for detection of light originating from scintillator crystals. Historically a limitation of SPM’s, with respect to their use with LaBr3 or CeBr3 scintillator crystals, has been their poor detection efficiency at blue wavelengths, which is the range for the fluorescent emission peak of both CeBr3 (370nm) and LaBr3 (380nm). In order to address this, in parallel with the CeBr3 scintillator development program, a pixelated SPM, with a high efficiency blue light response, has also been developed by SensL Technologies Ltd, under contract to ESA. The response observed from the SPM when coupled to different scintillator crystals and exposed to a 60Co source for 300 seconds can be seen in Figure 5. The crystals used, NaI, CeBr3 and

LaBr3, were 2” (NaI, CeBr3) and 1.5” (LaBr3) crystals and all measurements were conducted under the same conditions.

Fig. 5. The measured spectral response when coupling NaI, CeBr3 and LaBr3 to the Si-PMT and exposing it to a 60Co radiation source.

A clear shift in peak position to higher channel number can be seen when going from NaI to TlBr3. This corresponds to the relative light yield for the different materials. For all three crystals a high number of counts where registered showing that the SPM does indeed have a high blue light efficiency. The energy resolution for both CeBr3 and LaBr3 with PMT readout, was found to be ~4% FWHM at 662keV. Further enhancements of the SPM device are expected to yield higher spectral resolution.

VI. SUMMARY AND CONCLUSIONS

Large volume CeBr3 crystals intended for use as Ȗ-ray scintillator detectors have been developed under a current ESA contract. Crystals up to 3” in size have

successfully been grown. A spectral resolution of 4.1% at 662 keV has been achieved, which is slightly worse than that obtained for LaBr3 (2.8% FWHM at 662keV). The cause of the lower energy resolution is currently believed to be due to a higher degree of non-proportionality in CeBr3. The measured internal activity show that CeBr3 has an almost 30 times lower noise level then LaBr3, making the crystal very well suited for low noise applications. In a parallel ESA contract, a large area SPM array with a high blue light efficiency has been developed. Measurements carried out when coupling the SPM to both LaBr3 and CeBr3 have demonstrated excellent blue light response.

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