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The Medium Energy Instrument

The Medium Energy (ME) instrument consisted of an array of eight proportional counter, with a total geometric area of 1600 cm2 and 45 arcmin FWHM field of view, providing spectral and temporal data in the 1-50 keV energy range.

Each proportional counter had two gas chambers separated by a 1.5 mm beryllium window with argon in the top layer and xenon in the lower. Each proportional counter used a multi-wire design with a ∆E/E of 21% (6 keV) FWHM for the argon chambers and ∆E/E of 18% (22 keV) FWHM for the xenon chambers. The argon and xenon spectra were pulse height analysed into 128 channels each, sensitive to 1-20 keV and 5-50 keV energy range respectively.

The ME background was very stable and dominated by particle events from the solar wind and radioactive lines caused by the decay of residual plutonium in the Beryllium windows and detector bodies. The background count rate per detector were in the argon chamber 2.4, 4.3 and 9.4 counts-1 in the energy range 1-6, 1-10 and 1-20 keV, respectively, and 40.6 and 59.1 counts-1 in the xenon chamber in the 10-30 and 10-50 keV energy range, respectively. Occasional background flares occur in both the aligned and offset halves caused by enhancements in the solar wind. A major solar storm typically happened every six months, causing the background to increase by several order of magnitude. The detectors were turned off during these events.

To optimise the background subtraction each half of the ME array detector could be offset from the source, pointing a source-free region of the sky to monitor the particle background. The offset half was alternated every few hours (known as 'array swap'). Since the background obtained from the offset half was slightly different for the half of the detector on source, Difference spectra were created to correct this effect. Background was also obtained using the slew on and/or off the source. This technique was used when the detector halves were coaligned and no array swaps made during the observation.

In 1985 August 20 one of the detectors in half-1 failed. Occasionally because of small detector breakdown and/or reduction in the gain, observations were carried out with one or more detectors off for a few hours.

An important component in the operation of the ME instrument was the usage of the OBC. Depending on the objective of the observation, the OBC programs traded time resolution against spectral information and telemetry load. Two or three ME programs were typically run simultaneously. The table gives an overview of most commonly used ME OBC mode.

Program Output Time resolution No of PHA Channels Comment


10, 5 or 2.5s
150, 125 or 31ms

128 + 128 det, quad, or half
summed over Ar+Xe
HER5 Spectral
selectable only half
summed over Ar
PULS Spectral fold over period selectable for fast pulsars
HTR3 Intensity 4,8 or 16ms Ar and/or Xe low OBC usage
HTR4 Intensity 0.25ms 1 selectable channel
HER6 Intensity typically 4ms 1 or 2 selectable channel
HER7 Intensity typically 4ms 1, 2 or 4 selectable channel

A typical ME observation was carried out with a primary spectral oriented program and a secondary high time resolution program. The OBC introduced a count rate dependent deadtime due to the limited data sampling.

Last Update: 24 August 2005

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