Status:PRELIMINARY

You should check the parameters, that tell you something about the beam properties. First there is the beam current in the `Faraday cup'. It should be stable, but may slowly drift (usually up) over several hours. Of course, we may sometimes deliberately choose to run with different beam current especially if we change the target.

The `ladder OR16' rate is the count rate for the OR of 16 tagger channels (divided by 10), so is connected to to total tagged photon count rate. THIS SHOULD NOT EXCEED THE PERMITTED LIMIT (about 800000 - see the sheet on the window between the two parts of the counting room. Exceeding the permitted rate will burn out the tagger phototubes! It should also be stable, but will follow the Faraday and P2 ion chamber rates.

Then there is the `P2' ion chamber value. It is proportional to the photon rate measured at the end of the photon beamline after having passed through the target.

If all three values are changing, this may indicate changing beam intensity. You may call the operator and ask him, if he also noticed any changes or problems.

You should also call the operator, if the ladder/P2 ratio is rises significantly and you couldn't optimize it. (But note that, because of deadtime in the divide by 10 unit, this ratio depends slightly on the beam current.) Increased ladder/P2 ratio may indicate poor electron beam properties and an optimization of the beam by the operator might be necessary. Before requesting this make sure:

1) tagger HV is turned off (remotely - see 'tagger HV')

2) the (photon) beam stop is in.

3) The radiator is out (ie. blank in) - GET THE BEAM OFF before doing this!

(Warning: nightshift operators sometimes struggle with optimisation, so if it is not too bad it may be better to wait until daytime for electron beam optimisation.) If the ratio changes within 10$\%$, then you can possibly try to optimize it yourself (see 'electron beam steering'.), though this change isn't critical.

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Dan Watts

Last modified: Fri Jan 18 16:32:45 GMT 2002