A 320-keV Spectrometer Based on 8-Pixel Transition Edge Sensor With Trilayer Membrane and Novel Numerical Analysis

A 320-keV spectrometer based on an 8-pixel transition edge sensor (TES) has been developed. It involves a unique 7-μm thick membrane consisting of silicon dioxide (SiO
₂
), silicon nitride (Si
_x
N
_y
), and SiO
₂
. This membrane supports a 0.8-mm thick Sn absorber with suitable stopping power for photon energy
E
= 320 keV and is applicable up to
E

$ \approx $
0.7 MeV. Our trilayer membrane exhibits the same thermal conductance as conventional 1-μm thick Si
_x
N
_y
one, i.e.,
G

$ \approx $
1.2 nW/K at the critical temperature of 115 mK. We carry out the spectroscopy using a
⁵¹
Cr radiation source and simultaneously read out the 8 pixels with a microwave SQUID multiplexer. We find a linear broadening trend between the pulse peak and pulse area, even after a correction of the baseline drift due to time-varied temperature of our cryocooler. Based on the relationship, we analyze the data and succeed in avoiding degradation of the energy resolution in spectra. The resultant full-width-half-maximum (FWHM) energy resolution Δ
E
_FWHM
of the measured 6 pixels satisfies Δ
E
_FWHM
<1.6Δ
E
_NEP
where Δ
E
_NEP
is the noise-equivalent-power energy resolution. The best energy resolution among them is Δ
E
_FWHM
= 159 eV at 320 keV. This is 5 times as high as that of high-purity germanium detectors at 77 K and the best resolving power for gamma-ray TESs operated at
E
>220 keV to our knowledge.