1 X-Ray  -Calorimeter R. Kelley NASA/GSFC July 19-20, 2007 SRON

2 Status in US Proposal to NASA for participation in SXG submitted (April 2007). –Expect results by September Rashid Sunyaev visited NASA HQ and Goddard on June 22. –Very useful visit! We have embarked on evaluating our spare XRS arrays that would be used for SRG and also established collaboration to obtain new absorbers (HgCdTe) with size appropriate for SXG.

3 We have a number of spare detector arrays. These will need to have HgTe absorbers attached. One spare detector assembly that is assembled, but would need modification to operate at 50 mK (change in bias resistors.) One engineering unit Calorimeter Analog Processor (CAP) and one Calorimeter Digital Processor (CDP). We also have one engineering unit of the ADR Controller and HK electronics (ACHE). These were not designed for spacewire and would require an interface box. Existing XRS Spare Hardware

4 Progress on Detectors - Absorbers: For much of our work we have used HgTe for the x-ray absorber. Has relatively low  D (~ 150K), but thermalizes x-rays well. But we have observed a linear term in the heat capacity, identified as an electronic heat capacity, that typically exceeds the lattice heat capacity by a factor of ~ 3 at 60 mK. We associate this with defects, e.g., Hg vacancies, that act as dopants and hence provide enough carriers for the electronic specific heat to dominate. If we could remove the defects, such as by annealing in Hg vapor, we could potentially remove, or at least reduce the magnitude of the linear term.

5 Another approach is to reduce the number of states available for free carriers. HgTe has what can be thought of as an overlapping band gap structure. The band gap structure changes with the addition of Cd. With a value of x = (at T =0), the band gap of Hg 1-x Cd x Te becomes zero. x=0x=0.166

6 The electronic contribution to the specific heat of Hg Cd Te is expected to be substantially lower than that of HgTe. This is due to the lower electron effective mass, m*, of Hg Cd Te, which leads to a lower conduction band density of states, and hence a lower value of  in the expression C =  (T/  D ) 3 +  T Using the computational approach of M.E. Flatté, C.H. Grein, H. Ehrenreich, R.H. Miles, and H. Cruz, J. Appl. Phys. 78, 4552 (1995), Grein, Zhao & Sivananthan at the University of Chicago at Illinois obtain x=0.166  x10 reduction in m*. Hence, Hg 1-x Cd x Te with x = 0.166, combined with Hg annealing, should have a significantly lower heat capacity. But perhaps thermalization will not be as good. We have embarked on a collaboration with EPIR Inc. to provide Hg 1-x Cd x Te samples to evaluate this idea.

7 Hg 1-x Cd x Te, x = 0.16, 790  790  6  m MnK  1,2 profile FWHM = 4.2 eV 50 mK heat sink

8 Have not finished characterizations to infer heat capacity. Some of the improvement relative to XRS is coming from operating at 50 mK compared with 60 mK. But we know that the HgCdTe thermalizes well EPIR can provide what we need for SXG