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Nuclear Instruments and Methods in Physics Research A 520 (2004) 472–474

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doi:10.1016

Cosmic ray effects in microcalorimeter arrays

C.K. Stahlea,*, K.R. Boycea, G.V. Browna,b, J. Cottama,1, E. Figueroa-Felicianoa,M. Galeazzic, R.L. Kelleya, D. McCammond, F.S. Portera,

A.E. Szymkowiake, W.A. Tillotsona,b

a NASA Goddard Space Flight Center, Greenbelt, MD 20771, USAb University of Maryland, College Park, MD 20742, USA

c University of Miami, Department of Physics, Coral Gables, FL 33146, USAd University of Wisconsin, Physics Department, Madison, WI 53706, USA

e Yale University, Physics Department, New Haven, CT 06520, USA

Abstract

We have identified signals resulting from cosmic rays and environmental gamma rays depositing energy in the pixels

and in the silicon frame of the Astro-E2/X-Ray Spectrometer microcalorimeter array. Coincidences between pixels and

between the array and an anti-coincidence detector determined the nature of the events. Pulse shapes and amplitudes

from the cosmic ray events helped refine the thermal model of the array chip. We discuss how future arrays can be

optimized either for the greatest background rejection or for the highest source count rates.

r 2003 Elsevier B.V. All rights reserved.

PACS: 95.55.Vj; 95.55.Ka; 85.30.�z; 07.20.Fw

Keywords: Microcalorimeter array; Cosmic rays; Thermal detectors

1. Introduction

The X-Ray Spectrometer (XRS) microcalori-meter array [1,2] is a close-packed 6� 6 array ofindividual calorimeter pixels centered on a12 mm� 15 mm Si chip. Each pixel contains a(0.624 mm)2� 8 mm HgTe absorber attached to anion-implanted thermistor formed in a 1.5-mm thickdevice layer. Each thermistor is suspended above aseparate well etched into the substrate. We mountthe array over a cut-out in an alumina fan-outboard with Stycast 2850FT.

onding author. New name: Caroline A. Kilbourne

1-286-2469; fax: +1-301-286-1684.

ddress: cak@lheapop.gsfc.nasa.gov (C.K. Stahle).

RC Resident Research Associate.

- see front matter r 2003 Elsevier B.V. All rights reserve

/j.nima.2003.11.376

We have seen occurrences of simultaneoussignals on a large number of the pixels in thisand earlier arrays. In the XRS configuration, weoperate a silicon ionization detector directlybehind the calorimeter array for particle anti-coincidence [3]. This 1 cm2� 0.5 mm detector iscentered below the array in the hole of itsalumina board. Using the anti-coincidence (anti-co) detector we identified background cosmicrays and gamma rays as the origin of the multi-pixel events. The study of these events, incombination with heat sinking measurements,alpha particle irradiation, modeling individualpixel response, and thermal cross talk measure-ment, helped refine the thermal model of the wholearray chip.

d.

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C.K. Stahle et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 472–474 473

2. Heat sinking measurements on test arrays

To measure the heat sinking of the detector chipand its alumina board, we used resistors implantedinto the frame of a test array as heaters andepoxied additional heaters and thermometers tothe alumina board. We regulated the adiabaticdemagnetization refrigerator (ADR) at 60 mK andapplied power to the various heaters. We asso-ciated changes in pulse height or thermistorresistance with temperature by noting the ADRtemperature required to reproduce those changesin the absence of applied power.

The alumina board is thermally anchored to agold-plated detector box through Au wire bondswith typical total conductance of 10 mW/K. Thedetector chip is sunk to the alumina only throughthe bonding epoxy. We estimate the epoxy bondline to be about 0.15 mm wide and 25 mm tall.The thermal conductance of this bond at 60 mKwas 90 nW/K.

We used the 5.5 MeV alpha particles from anAm241 source to simulate cosmic rays. We admittedthe alpha particles through a 6-mm-thick film ofaluminized mylar (to block IR photons) over anoffset aperture. We estimate the maximum energyof an alpha particle reaching the silicon frame ofthe array to be about 4.5 MeV. We monitored fournon-adjacent pixels for simultaneous pulses. Thepulse heights of coincident pulses ranged continu-ously up to a height that would correspond to1.7 keV X-rays. Using a thermal model of a pixeland adding another temperature stage for theframe and its measured thermal conductance tothe alumina board, we can reproduce these frameevents. The model requires a heat capacity for theframe that is a factor of 20 higher than theDebye heat capacity for the silicon alone. Frameevents appear similar to absorber events due to thefact that the absorber-to-thermistor, the pixel-to-frame, and the frame-to-board time constants areall of the order of a millisecond.

Fig. 1. XRS pulse energy versus time, showing many coincident

events. The gap is due to the trigger threshold. Events below the

gap are due to electrical crosstalk.

3. Cosmic ray effects in complete arrays

During laboratory testing, we observed theresponse of XRS flight candidate arrays to cosmic

rays and environmental gamma rays. Fig. 1 showsoptimally filtered pulse height versus time for 32pixels in the XRS flight array. The differentchannels have been placed on a common energyscale based on fitting a gain curve to a series ofX-ray fluorescence lines used for calibration.Vertical lines of dots representing multiple pixelswith simultaneous events appear with a rateincreasing with decreasing energy.

The XRS calorimeter digital processor (CDP)[4] assigns to each pulse event a pulse height,arrival time, and several flags. The anti-co flag isset if a calorimeter pulse occurs within a set timewindow with respect to an anti-co pulse. We used a0.4 ms wide window and set the anti-co thresholdto a level corresponding to about 17 keV. Thearrival times of correlated pulses were spread overa range of about 0.4 ms. We defined as a singleframe event any series of calorimeter pulses fromat least seven different pixels with each pulseoccurring no more than 100 ms before the nextpulse. We associated an anti-co event with a frameevent if any pulse in the series had an anti-co flag.We can also correlate an individual pixel eventwith either a frame event or an anti-co event. Theindividual pixel event is distinguished from thepulses in a frame event by a substantially largerpulse height than the other coincident pulses. Inorder definitively to distinguish minimum ionizingparticles from gamma-ray Compton scatteringevents, we examined triple coincidences between

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Fig. 2. Cross section of segment of microcaloriemeter array

and anti-co, showing how minimum ionizing particles can

deposit energy in the absorbers, the frame, and the anti-co.

C.K. Stahle et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 472–474474

a pixel, the frame, and the anti-co. Fig. 2 shows thegeometry of the coincident events.

We typically measured 5.5 frame events/min. Aquarter of the frame events were associated withanti-co events. We selected from these events onlythose that had a pixel pulse 50% bigger than thenext biggest pulse in the coincident group. Thisthreshold was chosen to be above the statisticalclustering of the coincident pulses. Normalizing thetriple coincidence rate to the total HgTe area, we get1.2/min/cm2. Due to the hole in the frame behindeach pixel, 7% of particles passing through anabsorber do not deposit energy in the frame. Thegeometrical overlap between the pixels and anti-co,given the anisotropic flux and the vertical orienta-tion, is 99%. Thus the adjusted rate of triplecoincidences is 1.3/min/cm2, which is consistent withpublished values for the sea level muon flux [5,6].

For analysis of pulse records on a common timebase, we intercepted some signals before the CDPand recorded them with a four-channel digitaloscilloscope. We observed differences between thepeaking times of simultaneous pulses of up to0.3 ms. The relative peak times of the fourchannels changed with each event, providing arough indication of the position of the frameimpulse. A large component of the apparent extraheat capacity in the frame must be distributedthroughout the frame because of the large range ofrise times seen for the same frame impulse. The

degenerately doped contacts should only contri-bute a term comparable to the Debye term, thusthe source of this heat capacity is unknown.

4. Discussion

The size of the XRS array frame was chosen toallow for large bond pads and an easy scheme formounting the array above the anti-co detector.Although, with the anti-co, it provides almostperfect background rejection, it is far bigger thanthat task alone requires. At 1.6 cm2, the rate offrame event pulses per pixel in low-earth orbit willbe about 2/s. This provides a quiescent count ratethat must be added to source counts in calcula-tions of dead time. If we improved the heat sinkingof the outer parts of the frame through applicationof Au cooling pads and Au wire bonds, we wouldbe left with enough sensitivity to cosmic rays in theframe around the pixels to make a good anti-cowithout the high quiescent rate.

If cosmic rays hitting the frame make pulses thatlook like X-rays, then X-rays hitting the frame willlook like optical photons, and thus will contributeto the noise. We have measured a rate dependentnoise in the XRS array that is partially alleviatedby restricting the size of the aperture. Theremaining rate dependent contribution is consis-tent with pixel-to-pixel thermal crosstalk. For thebest performance for high rate observations, theentire frame needs to be well anchored. For lowrate observations for which particle background isa greater problem, a slightly decoupled frameprovides useful particle rejection.

References

[1] C.K. Stahle, et al., The next generation microcalorimeter

array on XRS of Astro-E2, Nucl. Instr. and Meth. A, these

proceedings.

[2] R.P. Brekosky, et al., Fabrication process responsible for

fundamentally improving silicon X-ray microcalorimeter

arrays, Nucl. Instr. and Meth. A, these proceedings.

[3] C.K. Stahle, et al., Proc. SPIE 3765 (1999) 128.

[4] K.R. Boyce, et al., Proc. SPIE 3765 (1999) 741.

[5] S. Tsuji, et al., J. Phys. G 24 (1998) 1805.

[6] K. Hagiwara, et al., Phys. Rev. D 66 (2002) 01000.1.