ASIM Scientific Instruments

ASIM scientific instruments include 2 cameras, 3 photometers and one X- and gamma-ray detector. The 2 cameras, 2 photometers and the X- and gamma-ray detectors are directed downwards (nadir).

The cameras and the photometers constitute the so called Modular Multispectral Imaging Array (MMIA). The final MMIA module will point to the nadir. The MMIA instruments will observe in different optical spectral bands.

The X- and gamma-ray detector is called the Modular X- and Gamma-Ray Sensor (MXGS). X- and gamma-rays are strongly absorbed in the atmosphere. This is why the detector points directly downwards, such that a minimum of atmosphere is between the detector and the thunderstorms within its field of view. Most of the atmosphere is below the altitude where giant lightning and terrestrial gamma-ray (TGF) flashes are generated. Therefore, space is particularly well suited to observe these phenomena in the band reaching from gamma-rays to UV, which is difficult to observe from the ground. ASIM is measuring in these bands.

 

MMIA (Modular Multispectral Imaging Array)

The optical instruments are a group of two optical narrow band CCD cameras and three photometers looking downwards towards the nadir with related optical and signal processing capabilities including autonomous event detection algorithms to identify and prioritize events for download.

 MMIA

Figure 4: The nadir-viewing assembly (MMIA) of 2 cameras + 2 photometers + MXGS (right). The final MMIA module includes an additional photometer. (Image credit: DNSC).

The cameras and photometers are equipped with baffles for stray light protection. The camera sampling is 12 bit 1024 x 1024 pixel frames at a maximum of 25 Hz.

 

PARAMETER CAMERAS PHOTOMETERS
FOV
(Field of View)
61.4º x 61.4º
(nadir direction)
61.4º x 61.4º
(nadir direction)
Detector 1024 x 1024 pixels
frame type CCD
 
Spatial
resolution
300 - 400 m
(nadir direction)
 
Data
quantization 
12 bit 12 bit
Time
resolution 
65 ms 100 kHz
(temporal sampling) 

Table 1: Parameters of the MMIA optical instruments 

The photometers are used to measure rapid time variations, which cannot be done by the imaging cameras. They view the exact same region but measure only the total photon flux from the region - but with high time resolution. The photometer FOVs are identical to those of the cameras: 61.4º x 61.4º (nadir direction).

 

Band

CAMERAS

Band

PHOTOMETERS

Spectral band(nm)

Bandwidth (nm)

Spectral band(nm)

Bandwidth(nm)

 

 

 

NP1 (nadir)

145 - 230

broadband

NC1 (nadir)

337.0

5.0

NP2 (nadir)

337.0

5.0

NC2 (nadir)

777.4

5.0

NP3 (nadir)

777.4

< 10

  Table 2: Optical parameters of the MMIA instruments

 

MXGS (Modular X-ray and Gamma-ray Sensor)

The MXGS is designed to detect bremsstrahlung from TGFs (Terrestrial Gamma Flashes) and from lightning induced electron precipitation (LEP). The detector is based on a CZT (Cadmium Zinc Telluride) semiconductor detection plane of 1032 cm2 (32 cm x 32 cm) with possible imaging capabilities. This detector is characterized by a large stopping efficiency for X and gamma rays up to a few hundred keV. The detector energy sensitivity ranges from 7 to 500 keV with an energy resolution of < 10% @ 60 keV.

 

PARAMETER

Low-E

High-E

(extension under consideration)

Energy range

10 – 20 keV

0.2 – 10 MeV

Effective area of detector

1032 cm2

900 cm2

Energy resolution of detector

< 10% @ 60 keV

18% @ 662 keV

Efficiency

> 90% @ 100 keV

> 60%

Imaging
(extension under consideration)

< 2º

 

 Table 3: Technical parameters of MXGS 

The MXGS detector plane consists of a 1024 cm2 array of CZT detector crystals. It is protected against the background radiation by a passive graded shield surrounding the detector housing. A hopper shaped collimator defines the 80º x 80º field of view for MXGS and shields the detector plane against the Cosmic X-ray Background. The DFEE (Detector Front End Electronics) is mounted in the housing below the detectors. The electronics contains also the HVPS (High Voltage Power Supply) and LVPS (Low Voltage Power Supply) as well as the DPU (Data Processing Unit).

The DFEE design consists of 4 DAUs (Detector Assembly Units), and each DAU consists of 16 DM (Detector Modules) and one DAB (Detector Assembly Board). The DAB holds the read-out electronics and the RCU (Readout Control Unit). The RCUs interface to the DPU. The purpose of the DAU is to read out the events and transfer the data to the DPU. 

The DM consists of two separable units, a CZT sensor and an ASIC. The sensor comprises four 20 mm x 20 mm x 5 mm CZT detectors tiled together on a PCB. Each detector is pixelated into 64 pixels (2.5 mm pixel pitch), making a 16 x 16 pixel array in total. The detector unit is stacked onto the ASIC unit via three connectors.

The MXGS uses fast ASICs to provide the time history and spectra over the course of the expected TGFs lifetime of 1-5 ms and a TGF burst trigger signal is passed to the companion MMIA module (and visa versa). The observation plane is protected from background radiation by a passive shield and the field of view is defined by a hopper shaped collimator.