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Changes to the ME CCF

The ME CCF has recently been updated to include the gain change parameters (
Express No. 6 p.25) and background difference spectra supplied by Alan Smith. Users can identify the new CCF by a layout number of 4 for the CB (Channel Boundary) data type and by the presence of a new data type DS (Difference Spectra).

The 1st record of the CB data type id unchanged and the 2nd has the following layout:

0-63SHF key of Gain measurements time.
64-127SHF key of start time of applicability
of gain drift parameters.
128-159Gain drifts, units are per day*106.
160-191Digital Ag's at time of gain measurement.
For a description if the above parameters and how to apply them see Express No. 6, p.25. Note that the gain measurements currently in the CCF were made on 1984 day 86 (SHF=133574400) and that there is no evidence for any Xenon gain changes.

The layout of data type DS is as follows:

1Argon counter A, configuration (a) spectrum
2Argon counter A, configuration (b) spectrum
3Argon counter B, configuration (a) spectrum
4Argon counter B, configuration (b) spectrum
17Argon half1 configuration (a) spectrum
18Argon half1 configuration (b) spectrum
19Argon half2 configuration (a) spectrum
20Argon half2 configuration (b) spectrum
21-40The same for Xenon
Note that each record contains data for 128 channels. In order to obtain spectra in cts/sec/bin the values stored in the CCF must be multiplied by 10-5. Note that there are no data in the records appropriate to configuration (a) since this is not normally used. Uncertainties are not included in this data type pending investigation of the systematic errors associated with the spectra.

Each of the spectra represent the difference in background counting rate obtained when a detector is fully offset in the +ve direction compared with that obtained when it is co-aligned. See Express No.5 p.48 for a detailed description of background subtraction techniques and difference spectra. These difference spectra should be subtracted from a background spectrum obtained in a fully positive offset configuration and added to a background spectrum obtained in a fully negative configuration prior to subtraction of the background spectrum from an aligned spectrum to obtain the source spectrum.

Note also that an error was introduced into some recently produced CCF's. Two of the configuration (b) Argon detector C resolution coefficients had become transposed. to check if this is the case on a particular CCF compare the configuration values (a) and (b). If they differ, then the configuration (b) values are wrong! Current CCF's (identified by a time of SHF=152634849) are correct and also include a better estimate of the detector D Argon gain drift parameters determined in the recent Crab Nebula calibration.

A.N. Parmar

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