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Wide Field X-ray Monitor

The WXM consists of two identical units of one-dimensional position sensitive X-ray detectors. They are oriented orthogonally to each other to measure the X and Y directions independently.

One WXM unit consists of a one-dimensional coded mask and two 1-D position-sensitive proportional counters (PSPCs) placed 187 mm below the mask. The coded mask is made of a plate of aluminum (0.5mm thickness) and gold (0.025mm) with a series of slits of randomly varying width (random mask). The location of the GRB is determined by measuring a set of two shift distances of the mask pattern in the X and Y direction. The mask pattern of WXM is selected from 100,000 random patterns with the same open fraction and element size based on localization accuracy for GRBs.

Each PSPC has three carbon fiber anode wires (10 micron diameter) in its upper cells and four tungsten wires in its lower cells, and filled with 1.4 atm Xenon gas with 3% CO2 as quenching gas. The three upper cells are used for X-ray detection. They have a depth of 25.5 mm and a width of ~27 mm, and are separated by the cathode wires placed at intervals of 3.4 mm. The four lower cells are used for rejecting charged particle events by anti-coincidence, and have a depth of 11.5 mm. The detection area of 120x83.5mm2 at the top side is sealed by 100 micron thick Be windows.

The position of an incoming X-ray is determined by the charge-division method. That is, L/(L+R) has a linear relation with the incident position, and L+R measures photon energy. Here, L and R represent pulse height values measured at left and right ends respectively.

The proportional counters and electronics are provided by RIKEN, while the coded aperture and on-board localization software is provided by Los Alamos National Laboratory.

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The left image shows the WXM proportional counter. This is encased in the middle portion of the spacecraft, in the rectangular section immediately below the canted wall section.
Built by RIKEN (Japan) and Los Alamos National Laboratory
Instrument type Coded Mask with Position Sensitive Proportional Counter
Energy Range 2 to 25 keV
Timing Resolution 1 ms
Spectral Resolution ~22% @ 8 keV
Detector Quantum Efficiency 90% @ 5 keV
Effective Area 175 cm 2 for each of two units
Sensitivity (10 sigma) ~8x10 -9 erg cm -2s -1 over the 2-10 keV range
Field of View 1.6 steradians (FWZM)
Angular resolution +-11 arcmin (normal incidence, 8 keV)

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