MPSD-8+(Data sheet V5.0_01)

Frontend readout module
for position sensitive
neutron detector tubes

More and more modern experiment setups in neutron spectroscopy take advantage of new position sensitive detector tubes, which provide 6mm position resolution over a length of 1m with tube diameters in the range of
25mm to well below 10mm. The high position resolution allows setups getting closer to the sample and use a lar ger solid angle. This as well as the high peak rates of TOF spectrometers requires fast readout electronics allowing
to register data at the detector rate limit (about 100kHz per tube) with lowest possible dead time ratio.

Position resolution
TypeLength Diameter Resolution[mm][mm][mm] FWHM

RS / GE8bar 3He

350

25.4

2.5

RS / GE
12bar 3He

1000

86RS / GE
20bar 3He

400

83Toshiba

2000

25.425(6kΩ total)

resistive anode wires with 7kΩ/m•Position stability: Due to the close preamplifier
matching there is no measurable position shift
with temperature ( < 1chan @ dT =10K).

•Rate capability: 100kHz per tube, 800kHz continuous per MPSD-8+ module with 8 detectors
connected. Integrated data buffer for high peakrates. Very low position cross talk of neighboured events coming closely in time.

•Efficiency: very stable due to clearly settable
and stable amplitude thresholds.

•Time stamping at frontend: 100ns•

Bias supply: filtered, up to 3500V. Also well
suited for BF3 filled tubes. Preamp inputs withstand full discharges up to 2500V.

•Detector interfacing: high voltage coax cables(MPSD-8+ frontend connectors: SHV). Cable
lengths up to 3m have only minor effect on position resolution.

•Power consumption: 2.6W at ±6V, allows op-eration in vacuum.
•Data bus: Lemo cable, max 30m length
mesytec GmbH & Co. KG
Wernher-von-Braun-Str. 1, D-85640 Putzbrunn, Germany
phone: +49 - 89 / 456007-30 fax: +49 - 89 / 456007-39
info@mesytec.com - www.mesytec.com

MPSD-8+

Frontend readout module
for position sensitive
neutron detector tubes

Functional Overview
Readout path
The position sensitive detector tubes are read out ingroups of eight which are connected to a single NIM
module MPSD-8+. Each of the modules buffers theposition and amplitude data of the 8 connected detector
tubes and transmits them over a fast serial bus to acentral NIM module MCPD-8 (mesytec centralprocessing device). The event bus is physically a coax
wire. The MCPD-8 collects data of up to 8 MPSD-8+
or MSTD-16 modules, buffers the data and transmits it
via Ethernet to one (or more) data collecting PCs.
Linux based daq software mesydaq handles theincoming data. They are stored on harddisk anddisplayed live in histograms.

Rate capability, dead time
For a perfect readout electronics, the dead time of thesystem should only be determined by the detector
properties. The detector frontend electronics isdesigned to reject wrong positions when two neutron
events occur in the same tube within the signal decay
time of the first one (pile up rejection). For R&S tubes
with 1inch diameter and 8bar pressure, 3µs were foundto be the optimum dead time. Colliding events can berecognized when they are separated by more than
500ns and are then eliminated from data stream. For
thinner tubes with faster charge rise / decay time thedead time is shorter. Simulating a 1/2inch tube with8bar gas filling, the dead time is calculated to 2.8µs.

Transmitted event data
The MPSD-8+ modules provide simultaneous
amplitude and position information with 10bitresolution and a 48bit time stamp (100ns resolution).

The MPSD-8+ uses two very fast, precisely calibrated
converters to digitize position and amplitude of
8 detectors. This converter adds almost no dead time,
because the analog section can buffer the two
amplitudes of each of the 8 detectors for up to three
conversion times.

Remote control
The system offers a remote control data path back fromthe PC via ethernet and eventbus. It is possible toconfigure gains, thresholds and pulsers from the PC,
make pulser testruns, and store the complete
configuration in a file which is downloaded to theperipheral modules at the beginning of a new run.
Diagnostics
A built in test pulser, which feeds directly to thedetector inputs of the MPSD-8+, easily allows to checkall the signal path from preamplifier to the PC. The test
pulser, like all other functions, can be remotelycontrolled by software.
Time stamping
The MPSD-8 and MSTD-16 frontend modules use
their own time base (100ns step) to label the incoming
events in their buffer. When the events are sent to theMCPD-8 via event bus, the time delay from receiving
the event at MPSD-8 input to transmitting it to
MCPD-8 via bus is added to the event information.
The receiving MCPD-8 adds this time offset to create
the event time stamp.

An example for a TOF setup with fast detectors (3µsdead time): MPSD-8+ is receiving an elastic bragg
peak on one tube with 100kHz, the other tubes will
have a rate of 10kHz. So at the position of the bragg
peak, at the moment the elastic neutrons hit the detector (some us) this detector will have a detector induced dead time ratio of 30% and 0.1% converter deadtime. The other detectors of the same module at thesame time will have a detector dead time of 3%
and 0.1% converter dead time. After the elastic peak,
the rate will usually decay by orders of magnitude andthe dead times will be negligible. No additional deadtime is added through all the processing chain down tothe PC.

Input sensitivity (customer specific)
Can be varied by a factor of 4 by remote control.
Standard range: 6*10-13 C (gain=1) ± factor of 2
For highest position resolution: 2*10 -12 C ± factor of 2.
When high charge is required, the amplitude spectrumof the detector gets wider, so reducing gamma
discrimination and efficiency(higher threshold required).
Dark rate @ UHV = 2000V, no detectors connected,
1 hour of warm up, sensitivity 6*10-13C.
f < 1 count / hour per channel.

mesytec GmbH & Co. KG
Wernher-von-Braun-Str. 1, D-85640 Putzbrunn, Germany
phone: +49 - 89 / 456007-30 fax: +49 - 89 / 456007-39
info@mesytec.com - www.mesytec.com

2/3

MPSD-8+

Frontend readout module
for position sensitive
neutron detector tubes

Operation in vacuum

Mechanics

Outgasing, safe bias operation
The MPSD8 can be safely operated under vacuum
conditions from 10-1mb to about 10-5mb. The upperlimit (measured at 2.0kV) is due to HV discharge starting at this pressure. The lower limit is due to outgasing of PCB and cables.
The virtual leakage from PCB is about 5*10 -4mb*l/s
after 24 hours of pumping.

MPSD-8+ is available as standard NIM module or in a
housing for stand alone operation (e.g. mounted directly behind the detector bank).
In any case it comes in a 1/12 NIM size case (164 x250 x 34mm3 plus SHV connectors of 20mm).

Heating
The PCB temperature in a vessel with 25°C is 30°C
over most of the area and max. 40°C at a single
position near the ADC. The measured temperatures
allow safe operation in vacuum without specialcooling.

Internal building blocks
Preamplifiers
The detector charge signals are amplified by two low
noise charge sensitive preamplifiers, which arematched in gain to 0.1%. The preamp gain can be
adjusted via remote control to compensate for Detector
gain variations. The close thermal matching of
preamplifier pairs results in a very low position drift(less than 1 bin = 10-3 per 10°C) with temperature.
Pulse Shaping
The signals from the gain adjust stage are filtered bytrapezoidal shapers (1µs FWHM for 1 inch and 1/2
inch diameter tubes, typically 0.7µs for 8mm diametertubes).
The sum signal runs through a baseline restorer andthen to the window discriminator. The digital output ofthe discriminator is used to actively restore the sumand difference signal. This is essential at high rates toreduce "crosstalk" of subsequent neutron signals in thesame tube.
Then the signals of the both tube sides are digitized,
sum and position are calculated, and a time stamp isadded.

Standalone housing of MPSD-8+ in NIM case with mounting flaps

Pile up rejection
Also a pile up rejection is implemented. If two pulses
with same amplitude, one at the extreme left side, thefollowing at the extreme right side occur within 3µs,
the position shift of the second one will be lessthan 1%.
Only if the second pulse follows within less than 0.5µs
it can no longer be distinguished from the first one.
This comes very close to the theoretical limit.
mesytec GmbH & Co. KG
Wernher-von-Braun-Str. 1, D-85640 Putzbrunn, Germany
phone: +49 - 89 / 456007-30 fax: +49 - 89 / 456007-39
info@mesytec.com - www.mesytec.com

3/3