  We demonstrate so-called repetitive non-destructive readout (RNDR) for the
first time on a Single electron Sensitive Readout (SiSeRO) device. SiSeRO is a
novel on-chip charge detector output stage for charge-coupled device (CCD)
image sensors, developed at MIT Lincoln Laboratory. This technology uses a
p-MOSFET transistor with a depleted internal gate beneath the transistor
channel. The transistor source-drain current is modulated by the transfer of
charge into the internal gate. RNDR was realized by transferring the signal
charge non-destructively between the internal gate and the summing well (SW),
which is the last serial register. The advantage of the non-destructive charge
transfer is that the signal charge for each pixel can be measured at the end of
each transfer cycle and by averaging for a large number of measurements
($\mathrm{N_{cycle}}$), the total noise can be reduced by a factor of
1/$\mathrm{\sqrt{N_{cycle}}}$. In our experiments with a prototype SiSeRO
device, we implemented nine ($\mathrm{N_{cycle}}$ = 9) RNDR cycles, achieving
around 2 electron readout noise (equivalent noise charge or ENC) with spectral
resolution close to the fano limit for silicon at 5.9 keV. These first results
are extremely encouraging, demonstrating successful implementation of the RNDR
technique in SiSeROs. They also lay foundation for future experiments with more
optimized test stands (better temperature control, larger number of RNDR
cycles, RNDR-optimized SiSeRO devices) which should be capable of achieving
sub-electron noise sensitivities. This new device class presents an exciting
technology for next generation astronomical X-ray telescopes requiring very
low-noise spectroscopic imagers. The sub-electron sensitivity also adds the
capability to conduct in-situ absolute calibration, enabling unprecedented
characterization of the low energy instrument response.
