The MXGS Instrument

UiB have worked with the Norwegian firm Gamma Medica IDEAS (Now: IDEAS) on the development of the XA ASIC Module, and with the Canadian firm Redlen Technologies on the development of the CZT Module. Together these parts make the CZT DM which is one of the key components in the x-ray and gamma-ray sensor.

UiB have also worked with the Space Research Center, Polish Academy of Science (SRC PAS), Warszawa, Poland for designing the High Voltage and Low Voltage power supply (PSU) needed for the x-ray and gamma-ray sensor.

University of Bergen is responsible for developing and building the two X- and gamma-ray detectors for the MXGS sensor; the Low Energy Detector (LED) and High Energy Detector (HED). Cross-section of the MXGS instrument, showing (half a) LED and HED. (Illustration: UV). Cross-section of the MXGS instrument, showing (half a) LED and HED. (Illustration: UV).

Prototech have done the thermal and mechanical modelling for the HED and LED detectors as well as manufacturing of the mechanical structures in the HED detector.

The Low Energy Detector (LED) is a detector array of 8 x 8 (64) Cadmium Zink Telluride Detector Modules (CZT DM) with a dedicated read out electronics. LED is pixelated and with a detection area of 1024 cm2 it will measure energies in the range of 15keV up to 400 keV and enable imaging of the TGFs. The LED is built based on a modular structure comprised of 4 identical CZT Detector Assembly Units (CZT DAUs), see illustration below. LED – top view (Illustration: UB/UV). LED – top view (Illustration: UB/UV).

The High Energy Detector (HED) is 900 cm2 and consists of Bismuth Germanium Oxide (BGO) crystals coupled to photomultiplier tubes with its dedicated read out electronics. The HED detector will cover energies extending up to 20 MeV. The HED is built from 4 identical BGO Detector Assembly Units (BGO DAUs), see illustration below. HED – bottom view (Illustration: UB/UV). HED – bottom view (Illustration: UB/UV).

The scientific requirements for MGXS are:

Energy resolution

<10% at 60 keV (LED)    
<20% at 662 keV (HED)

Energy sensitivity range

15 keV to 400 keV (LED)
0.2 MeV to 20 MeV (HED)

Detection efficiency

>94% at 100 keV (LED)
>60% above 1 MeV (HED)

Burst event capability

1000 cts/ms

Background level

2.4 cts/ms (15-400 keV, LED)
2.0 cts/ms (0.2-20 MeV, HED)

 

For an MXGS total of 4.4 cts/ms.

Mounting

Nadir-pointing

Field of View

80 deg * 80 deg

Angular resolution of imaging

Better than 2 deg (LED)

Relative/absolute time accuracy

10/100 microsec

Trigger

Bi-directional link to MMIA

Point source Location

0.3 deg


It is expected that ASIM/MXGS will see about 1000 TGFs a year (ten times RHESSI and 100 times BATSE) and for each TGF we will have about 300 counts (ten times RHESSI).

The detector is protected against the background radiation by a passive shield and the field of view is defined by a hopper shaped collimator, see details in “ASIM Payload”

Time resolution and dead time effects

As TGFs are fast events, most of the photons arrives the instrument within 1 ms. The instruments that have detected them so far, BATSE, RHESSI, Agile and Fermi, were designed for other slower phenomena and they have therefore all suffered from deadtime effects. Special attention has therefore been given to ensure that MXGS will have the adequate time resolution. For the read-out system of the CZT layer we will use an Application Spesific Integrated Circuit (ASIC) developed by Ideas with a time resolution of 4 microsec for each 1/16 part of the layer. The BGO layer will have a time resolution of about 1 microsec for each 1/12 part of the layer. Confirmed by modelling results of deadtime effects, we are confident that MXGS will have a time resolution such that deadtime effects will be minimal, even for the largest TGFs.

The instruments that have detected them so far, BATSE, RHESSI, Agile and Fermi, were designed for other slower phenomena and they have therefore all suffered from deadtime effects.


With its sensitivity, spectral and temporal resolution MXGS will give unprecedented information about the time history and spectra of TGFs during their expected lifetime of 1-5 ms. The read-out system will also provide an event trigger to flag observations of bursts and to trigger the MMIA cameras.