The XRS Instrument

The detectors in XRS are X-ray microcalorimeters. They work by monitoring the temperature of a tiny piece of silicon, and measuring the temperature rise that results when it absorbs an X-ray photon.

You might imagine that measuring the temperature rise from a single photon is fairly difficult, and you'd be right! Briefly, here is how we do it:

• First, the X-rays must be focused onto the detectors. This is done with a set of conical mirrors made of hundreds of layers of very thin foil.
• The detectors need to be kept extremely cold (60 milliKelvin). This requires a complex cryogenic system, including liquid helium and solid neon. It also requires the use of several filters to keep out stray light, radio waves, and any other radiation other than X-rays.
• The signals from the detectors are amplified and shaped by a package of analog electronics and then processed digitally to determine the energy of each photon.

Here is a block diagram of the instrument (minus the mirrors), and a brief description of each subsystem. You can click on the diagram, or follow the links in the descriptions to learn more about the various subsystems.

FEA

The Front-End Assembly holds the array of 32 detectors at the focus of the X-ray mirrors, keeping them at a temperature of 60 milliKelvin (mK). The FEA also contains the first stage of signal amplification.

ADR

The Adiabatic Demagnetization Refrigerator controls the temperature of the FEA. This keeps the detectors at 60 mK and is stable to about 10 microKelvin.

Helium insert

The cryogenic insert (or "helium insert") contains the FEA and ADR, along with about 30 liters of liquid helium.

dewar

The dewar is an insulated container to keep everything cold. It consists of a jacket of solid neon, which surrounds the liquid helium filled insert. The heat from the detectors and amplifiers, plus the stray heat that leaks in from the outside, ultimately goes into melting the solid neon. When it is gone (after about 2 years in orbit), XRS will no longer function.

filters

These are a set of 5 extremely thin sheets of metal (only 3 are shown above) which let the X-rays through to the detectors, but keep out other radiation (such as ultraviolet, visible, or infrared light).

CAP

The Calorimeter Analog Processor powers the detectors and amplifies their signals by a factor of 20000. The amplified signals are sent to the CDP (see below).

CDP

The Calorimeter Digital Processor analyzes the amplified signals to determine the energy of each X-ray that hits a detector. The CDP digitizes the data from the CAP and monitors the data stream. When it detects the pulse from an X-ray, the signal is analyzed to determine the energy of the photon, and the resulting data is sent to the spacecraft for later transmission to the ground.

ACHE

The ADR Control and Housekeeping Electronics controls and cycles the ADR. The ACHE contains all the electronics for controlling the temperature of the ADR.

mirrors

(not shown above) The conical foil mirrors focus the X-rays onto the detectors.

spacecraft

XRS is a part of the Japanese ASTRO-E spacecraft. The spacecraft provides power and a stable thermal environment, as well as storing commands from the ground and data to be transmitted.