Speaker
Description
Summary
In the framework of the EU NMI3 project DETNI [8] a new generation of detectors for
imaging and time-of-flight applications with neutrons is under development. The
project targets the realisation of three different high rate, solid neutron converter
based detectors, one of which is a double-sided Silicon micro-strip detector, coupled
to a Gadolinium neutron-converter layer (Si-MSD). The other two detector types are
gas based: The "Cascade" GEM detector using a Boron converter and a Hybrid-MSGC
detector with Gd-converter. Either one of the detectors is characterised by a
comparatively high number of readout channels per coordinate (up to 640 x 640 for the
Si-MSD) and a specified event-rate, unprecedented in single event counting neutron or
X-ray detection technology.
To accomplish the readout of these detectors the 128-channel n-XYTER readout ASIC has
been developed. It will allow determining both, time of incidence as well as pulse
height of the detected signals. The former will allow localising the point of
conversion of neutrons through coincidence on both coordinates, the latter serves to
further enhance spatial resolution through centre of gravity calculation on one hand
and to realise X-ray background discrimination on the other hand. Particular
attention is paid to address the statistical nature of the incoming signals in the
purely data driven acquisition architecture.
Either one of the DETNI detectors is inherently capable of enormous neutron
conversion rates. The targeted rate capability for these detectors will entirely be
limited by the bandwidth of readout electronics and time resolution, which determines
coincidence resolution for position correlation. With an expected neutron conversion
rate of 100MHz on the detector, each signal needs to be tagged with a 2ns resolution.
Further, as neutron data is statistical in nature, and readout bandwidth limited, a
derandomization scheme together with a sparsification concept needs to be employed.
To achieve these goals, the chosen architecture implements a charge-sensitive
preamplifier for ultra low-noise and wide dynamic range. It drives two shaper stages
with different time constants per channel. The time constant of the faster shaper was
chosen just long enough to completely integrate the signal of a silicon sensor. It
drives a single-pulse discriminator with subsequent time-walk compensation. The
latter is required to restrict the amplitude-dependent time walk of the discriminator
to less than 2ns. The output of this circuit is used to latch a 14-bit time stamp
from a 500MHz grey code counter and to enable an auto-resetting peak detector circuit
fed by the slower shaper branch. The noise of the discriminator readout path is
expected to be only ~1000e with a 30pF detector, which together with threshold
correction on a per-channel basis will ensure a sufficient S/N ratio without
compromising efficiency. The analogue output of the peak detector as well as the time
stamp are fed to a 4-stage FIFO for derandomisation. The readout of these FIFOs is
accomplished by a token-ring based multiplexer working at 32MHz, which accounts for
further derandomisation, sparsification and dynamic bandwidth distribution. Due to
the unique properties of this architecture, implementations with a lower number of
channels, targeted for applications with even higher rates, are also planned.
We present the scientific output of an evaluative prototype together with the design
and implementation of the first full scale version. realised in silicon on the AMS
0.35um C35B4M3 process.
[8] (http://jra1.neutron-eu.net/jra1)
