SPI Coded Mask

The Spectrometer on board INTEGRAL (SPI) is a high resolution spectrometer designed to perform spectral analysis over an energy range between 20 KeV and 8 MeV with an energy resolution of 2 KeV at 1.33 MeV. The field of view of the Instrument is + 8º with an angular resolution of 2º.

Design, Manufacturing, Testing at Subsystem level and Delivery of SPI Mask is responsibility of the Astronomy and Space Science Group (GACE) of the University of Valencia, with the support of INTA for 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.


SPI Mask description


The principal role of the Mask is to intercept the gamma-radiation and project a coded shadowgram onto the SPI detector plane and it has been designed to carefully match the size and position of the Ge detectors. The SPI Mask is a Hexagonal Uniformly Redundant Array (HURA) made up of 127 hexagonal cells of which 63 are opaque to the radiation (95 % opaque @ 1 MeV) and 64 are transparent to gamma-radiation (60 % transparent @ 20 KeV & 80 % transparent @ 60 KeV). The pattern has a 120º symmetry and the cells are of size 60 mm side-to-side (i.e. short axis) by 67.55 ( i.e. long axis) forming a hexagonal-shaped coded area of overall dimensions 692.72 mm (i.e. short axis) by 770 mm (i.e. long axis). The opaque elements have been manufactured in a tungsten alloy (DENSIMET-18) 30 mm. thickness.

To meet the strength and stiffness requirements of the overall Mask. Assembly to overcome the launch environment and the in orbit operational activities, while maintaining the X and gamma-ray transparency of the open cells, the tungsten elements have been mounted on a carbon fibre structure. The support panel is made of Nomex Honeycomb and carbon fibre laminates in a sandwich construction. To provide a fail-safe design, the tungsten elements have been glued and bolted to the panel by means of titanium pieces, called inserts. The inserts have been integrated into the Nomex panel.


 The FM Mask on the Three-dimensional Machine at INTA (July 2000) Sandwich panel glued to the peripheral titanium ring.


The second important function of the SPI Mask is to act as a structural element to the whole SPI Instrument by providing structural continuity to the Anticoincidence System as well as by physically closing the tube structure thermally and electrically. Therefore the Mask panel is attached to a circular I/F ring of titanium alloy for bolting the mask to the Anticoincidence System by mean of 36 titanium screws equi-spaced 10º.

Four models of the SPI 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 CNES; the Flight Model (FM).

The Flight Model of the Mask has been delivered to CNES in November 2000 and it fulfils the high technical specifications. The test campaign was performed at Qualification levels on the STM model ( December 1998 – February 1999) and for the FM tests have been performed at Acceptance levels.

Concerning the Physical requirements the total mass has been 131.6 Kg ( less than 140 Kg. allowed). Other Physical Properties (MoI., CoG, and Electrical Continuity) are also within requirements.

To determine the Code real position several dimensional controls have been done during Manufacturing Phase and Acceptance Test campaign. The opaque elements have been manufactured in tungsten alloy by electro discharge wire (EDW) cutting with a + 0.025 mm accuracy in a 390 mm. The tolerance achieved in the Code Positioning respect to the Mask Reference System is + 0.02 mm by far within the + 0.15 required.

The design of the Mask has been qualified through Mechanical and Thermal Test performed on the STM. Levels: 17.5 g in Axial and 8 g. in Lateral. Temperature Qualification Range ( - 35º, + 40º).

Mask during TVT test at INTA facilities (July 2000).
 FM Mask on vibration table at INTA shaker (July 2000)


The scientific test (Pixels Transparency Measurements) on the FM Mask model were carried out from August to October 2000. The test has been performed using a dedicated set up designed by the University of Valencia Project Team and available at the UV Clean Room ( Class 1000). For Transparency determination, the 63 open pixels have been measured with several radioactive sources ( Am241, Ba133 , Na22) in different angles (0º, 1º, 2º, 10º) and energies ( 17 KeV, 31 KeV, 35 KeV, 60 KeV, 81 KeV, 302 KeV, 356 KeV) during 1 hour/measurement. For opaque grade determination 20 pixels have been tested ( 5 tungsten pixels and 15 tungsten pixels with inserts). The radioactive sources used were Na22 & Y88, the angles have been reduced to 0º ( on axis) and 10º ( off axis) and the energies reached were 511 KeV, 898 KeV, 1275 KeV and 1836 KeV.


See the image gallery showing more SPI coded mask pictures.

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