J Low Temp Phys (2012) 167:232–235 DOI 10.1007/s10909-012-0523-9

Kapton Polymeric Films to Shield X-Ray Detectors in Orbit E. Perinati · S. Lotti · L. Colasanti · C. Macculi · T. Mineo · L. Natalucci · L. Piro

Received: 29 July 2011 / Accepted: 10 January 2012 / Published online: 27 January 2012 © Springer Science+Business Media, LLC 2012

Abstract We are investigating possible technical solutions to minimize the instrumental non X-ray background (NXB) of the X-ray Micro-calorimeter Spectrometer (XMS) for the ATHENA space mission. In the proposed design, XMS will be provided with an anti-coincidence system in order to reject most of the X-ray-like events produced by primary solar and cosmic particles that are expected to populate the L-2 space environment. However, the rejection efficiency of events produced by secondary particles cannot be as good as that of events produced directly by primary particles. Among secondary emitted particles, knock-on electrons have in general a major impact in determining the NXB level of X-ray detectors. For this reason, it may be helpful to adopt some techniques of passive shielding together with the use of the active anti-coincidence. We present preliminary results of a study on polyimide sheets, which could be employed to reduce the fluence of knock-on electrons onto XMS and, more in general, to optimize the design and configuration of X-ray detectors in orbit. Keywords X-ray detectors · Instrumental background · Cryogenic

E. Perinati () IAAT—Institut für Astronomie und Astrophysik, Universität Tübingen, 72076 Tübingen, Germany e-mail: [email protected] E. Perinati · T. Mineo INAF—Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, 90146 Palermo, Italy S. Lotti · L. Colasanti · C. Macculi · L. Natalucci · L. Piro INAF—Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, 00133 Roma, Italy L. Piro Astronomy Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

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1 Introduction The X-ray Micro-calorimeter Spectrometer (XMS) [1] for the proposed ATHENA space mission will be devoted to perform non-dispersive high resolution imaging spectroscopy in the soft energy band 0.2–10 keV, with unprecedented sensitivity. To reach this goal, the instrumental non X-ray background (NXB) is required to be kept below 0.02 cm−2 sec−1 keV−1 . ATHENA will be placed in a halo orbit around the L-2 lagrangian point, where the geomagnetic field is not strong enough to effectively shield the focal plane from the bombardment of solar and cosmic particles. However, XMS will be provided with an anti-coincidence system capable to reject most of the X-ray-like events generated by primary charged particles. Nevertheless, a non negligible residual background will still arise from unrejected secondary particles, created in the interaction of primaries with materials and structures surrounding the detectors. Such particles are typically electrons and photons, but, depending on the actual adopted configuration of mass distribution around the detector, neutrons are to be taken into account as well. The rejection efficiency of secondary particles is lower than that of primary particles, since many produced secondaries have energy low enough to be fully absorbed in XMS. Therefore, it is useful to explore possible technical solutions to reduce in a passive way the fluence of secondary particles onto XMS. 2 Knock-on Electrons Preliminary GEANT4 simulations that we have performed in the framework of the ATHENA study indicate that, referring to the present design of mass distribution around XMS, including a cryostat and a number of internal metal plates, secondary electrons, after primary particle rejection by the anti-co [2], are the main source of NXB for the XMS instrument, producing ∼70% of all expected residual X-ray-like counts and determining a residual background level slightly above the requirement. Moreover, according to Monte-Carlo simulations secondary electron emission is actually recognized to have major impact on the NXB spectra observed by X-ray detectors in orbit, such as the Suzaku/XRS micro-calorimeter and the XMM/EPIC CCDs. Knock-on electrons are normally ejected from metals structures close to the focal plane and surrounding the detectors, under bombardment by ionizing energetic primary particles present in the space environment. This means, for example, that placing a metal shield around a detector to protect it from proton bombardment and possible displacement damages, which is important especially in the case of CCDs, implies the production of a flux of secondary electrons towards the detector, which can contribute to increase the NXB. Therefore, the individuation of techniques to reduce their fluence is a matter of interest for different types of X-ray detectors. 3 Polyimide Polymers One way to reduce the fluence of secondary electrons onto a micro-calorimeter or other type of X-ray detector (CCD) in orbit is by interposing a sheet with high electron absorbance capability between the detector itself and the main metal structures

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Fig. 1 Sketch of the geometry used in simulations

Table 1 GEANT4 computation of the emission rate of soft secondary electrons by a 10 micron thick shell (C, Al, Nb, Au, Kapton) under proton bombardment

Shell material

Rate of 0.2–10 keV electrons absorbed in the slab (cm−2 sec−1 )

Carbon

0.13

Aluminum

0.18

Niobium

0.29

Gold

0.42

Kapton

0.12

around it, in order to stop most of low energy secondary electrons ejected by them. At the same time, such material should have a low yield of secondary electrons self-emission. Therefore, we focused on polyimide polymers, which exhibit these properties. In particular, we selected Kapton and investigated its performances using GEANT4. Kapton is a polyimide film developed by DuPont [3], which is known to be stable in a wide range of temperature from 0 K to ∼700 K. It has also a low outgassing rate, which is important to avoid contamination of the detectors. Therefore, Kapton is suitable to be used in a cryogenic environment. We performed some simulations comparing the emission of secondary electrons in a few cases. We adopted a simple configuration with a spherical shell 10 μm thick containing a thin Bi slab at its center (see Fig. 1). For a few different shell materials, we shot 106 protons with the L-2 spectrum [4] onto the shell and counted the number of ejected electrons in the band 0.2–10 keV absorbed in the slab. Results (normalized to the slab area) are summarized in Table 1. We see that the yield of emission of secondary know-on electrons by Kapton is much lower than that of metals normally found in the instrument configuration, such as Al, Nb and Au, and even some lower than that of pure C. Therefore the insertion of a Kapton liner may help to reduce the fluence of knock-on electrons on the detector, since a few micron Kapton are capable to stop electrons up to 10 keV coming from a metal layer and in turn will emit a lower amount of knock-on electrons by itself. Of course, to really estimate the efficiency of the shielding we have to consider also all other secondary particles produced by the metal layer hitting the Kapton liner, so that the results will be dependent on the whole configuration around the detector. We have verified through a simulation that in the case of the configuration of the micro-calorimeter XRS on Suzaku, where the Au coating of the detector box is the

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material directly seen by the detector, the introduction of an insulating Kapton liner can reduce the residual NXB by a factor of ∼2. However, we remark that Kapton produces a strong fluorescence C K-line (277 eV) when bombarded by charged particles. Since this line lies just at the edge of the band of scientific interest, it should be not too critical and it may even be exploited to calibrate the detector gain in orbit. Another issue is the high heat reflectivity of Kapton, which has to be considered when the shielded environment has to be kept at very low temperature. A thermal characterization of Kapton at cryogenic temperatures will be then necessary before adopting it as a possible particle shield around a micro-calorimeter.

4 Conclusion We have found that the a Kapton liner deposited on the metal surfaces in view of an X-ray detector is an effective absorber of secondary knock-on electrons, that in many cases are the primary component of NXB. We plan to extend this analysis to the more complex ATHENA geometry, exploring also other passive materials, in order to optimize the design for the XMS instrument. The implementation of Kapton or polyimide materials as passive shields appears to be a very promising approach to reduce the residual particle background of the XMS microcalorimeter, and possible experimental tests may be planned in a suited facility to validate the predictions of Geant4 simulations. Acknowledgement

We acknowledge ASI under contract I/035/10/0.

References 1. 2. 3. 4.

J.W. den Herder et al., Proc. SPIE 7732, 77321H (2010) C. Macculi et al., These Proceedings LTD14, J. Low Temp. Phys. (2012) http://www2.dupont.com J. Kaastra et al., SRON internal report 05-004, 2005