JEM-X Coded Masks

The X-Ray Monitor on INTEGRAL (JEM-X) will perform observations with arc minute angular resolution in the 3–60 keV band. The instrument is based on the same measurement principle as the two gamma-ray instruments on INTEGRAL (SPI and IBIS): sky imaging using a Coded Aperture Mask.
The instrument consists of two high pressure Microstrip gas chambers that view the sky through two coded Masks located 3.4m above the detector planes. Both Masks are identical except for the 180º rotation of the Code.

Design, Manufacturing, Testing at Subsystem level and Delivery of JEM-X Masks are responsibility the Astronomy and Space Science Group (GACE) of the University of Valencia, with the support of INTA for the Thermal Design and Tests. Main contractor for the Mask development is the Spanish company SENER. About another 10 companies have taken part in the different activities


JEM-X Masks description


The JEM-X Mask code pattern is based on a Hexagonal Uniformly Redundant Array (HURA) of 22501 elements, with approximately 25% of them open. This transparency achieves better sensitivity than a 50% open code, particularly in complex fields with many sources, and reduces the number of events to be processed.

In order to obtain the required angular resolution (3.35 arc minutes) the dimension of the hexagonal cells has been chosen as 3.3mm side to side. The total number of hexagonal pixels is about 24000 for a total coded area of 535mm diameter, so a slight repetition of the code is needed.


JEM-X FM2 Mask
 JEM-X Mask Coded detail: "island"


The design driver was to provide a system able to survive the launch and operational thermal and mechanical conditions, maintaining the best scientific performances of the Mask. The main requirements are the following:

  • Technical Requirements:
  1. Strength (Sinus): 12g (Axial) 10g (Lateral)
  2.  Strength (Random): 0.07g2/Hz in 100-700Hz (Axial) and 0.05g2/Hz in 100-700Hz
  3. First Eigenfrequency >60 Hz (axial), 120 Hz (lateral)
  4.  Thermal Gradient: -65ºC + 40ºC
  • Scientific Requirements:
  1. Pixel Size Accuracy 0.05mm
  2. Pixel Position Accuracy 0.2mm
  3. Close Pixel Opacity >99.9%@35keV and 95%@60keV
  4. Open Pixel Opacity 100%

The JEM-X Mask is composed by four subsystems: Tungsten (W) Code, Titanium Ring, Cu-Be Pretensioning System and Exoskeleton (Strongbacks).

The Mask code is built on a 0.5mm thick Tungsten (W) sheet. This thickness provides the required opacities at the JEM-X energy band: >99.9%@35keV and 95%@60keV. The machining of the about 5800 hexagonal holes on a thin W sheet, within the scientific requirements (position and size of the pixels) is one of the technical goals of the Mask design.

 JEM-X Mask FM Code plateJEM-X Mask code machining by Electrodischarge Wire Machining at Mecanizados Ginés


The Interface with INTEGRAL PLM is defined by a Ti forged ring. This interface is provided by 12 equi-spaced holes. 44 Cu-Be elements pre-stress the machined Coded Plate to increase its stiffness and strength. The use of Cu-Be elements is quite innovative and reduces dramatically the need of any additional support structure.

Two strongbacks are located on both sides of the Code to reinforce the plate stiffness. These elements have been designed to minimise the occlusion of open pixels. Less than 2% of the open pixels are covered by the exoskeleton structure.

JEM-X Mask Pretensioning System
 JEM-X Mask mounting at GACE Clean Room


Two Optical Cubes are located on the Mask Ring in order to define an Optical reference of the Code for alignment of the Masks with the JEM-X detectors and INTEGRAL Line of Sight.

The Mask design has successfully passed the different thermoelastic and mechanical tests at Subsystem and Satellite level, providing a first axial eigenfrequency close to 200Hz, more than 3 times higher than required.

Four models of the JEM-X Mask have been developed between 1997 and 2000: Development Model (DM), for technology and manufacturing demonstration; STM/QM, built in flight standard and tested at full Qualification Level at INTA and Alenia; and two Flight Models (FM1 & FM2)

 JEM-X Mask Optical Cube Dimensional Control of the JEM-X FM Masks at INTA


JEM-X Masks FM1 & FM2 were assembled in Nov 1999 at GACE 1,000 class Clean Room by SENER and GACE engineers and passed successfully their Acceptance Test Campaign at INTA from November 1999 to January 2000.

Main results of the test on the FM Mask refers to the scientific performances of the Mask. The Mask optical capabilities as a Spatial Multiplexing System are defiend by the pixel size and positioning accuracy over the code. Several Dimensional Controls were performed on the FM Masks, with successful results: the values for pixels size and positioning uncertainty:

  • Pixel size uncertainty results:
  1. Requirement: 0.05mm
  2. FM1: 0.005mm
  3. FM2: 0.005mm
  • Pixel position uncertainty results:
  1. Requirement: DY=DZ=0.2mm
  2. FM1: DY=0.03mm; DZ=0.025mm
  3. FM2: DY=0.03mm; DZ=0.02mm

Average deviation in angular units is 2.2. arc sec for a focal length of 3.4m.

The Masks were accepted by DSRI & ESA in April 2000 and integrated on FM Satellite at Alenia in October 2000, being the first INTEGRAL Payload FM Hardware delivered to ESA.


See the image gallery for more pictures.


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