Obtaining a BCM Calibration

Obtaining a BCM calibration consists of two steps that cross calibrate the BCM's to the Unser Monitor. First the response of the Unser and the BCM's is measured at a series of beam currents that cover the range of beam currents to the current experiment. Second those measurements are analyzed to produce calibration constants for the BCM's.

Data Collection

The following is the recommended method of acquiring BCM calibration data. This procedure was designed for the Spring 2003 Hall C running. For other experiments the procedure may be modified, depending on the range of currents required by the experiment, but the procedure will be similar.

Goal

The goal is to perform a current monitor calibration over the range 10 to 100 $\mu$A.

Estimated Duration

1 hour

When

Soon after startup, but reliable high current beam is needed.

Impact on other halls

The calibration is normally invasive because, without turning off the other Hall lasers, it is the only sure way to get the required 0.000 microA needed for the Unser zero offset measurements.

1. The Run Coordinator needs to make pre-arrangements with the other halls since we're shutting them off for one hour.

2. Put in either of the 4 cm cryotargets. The choice of target isn't critical, but these are least likely to trip off the beam due to excessive dose rates in the ion chambers.

3. Find out if the MCC can deliver 100 microA. If they can't, the Run Coordinator needs to decide whether it's worth proceeding.

4. Make sure our data acquisition is working, and that BCM and CLOCK scalers are counting. Prescale away most spectrometer triggers so the daq is less likely to crash in the middle of the calibration.

5. Ask the operator to turn off non-Hall C lasers.

6. Start a "BCM Calibration Run" now before you forget.

7. Tell the operator your nominal current cycle will be:

   0, 10, 0, 20, 0, 30, 0, 40, 0, 50, 0, 60, 0, 70, 0, 80, 0, 90, 0, 100,

   and should then be repeated. Each current setting should be 1.5-2 minutes duration.

   If the green light is flashing on the scaler crate, you're probably taking data. The files are usually small enough to fit in the daq buffer, so the output .log file will be nearly empty until you finally stop the run.

8. (FINAL): After the calibration run is over, please replay completely, taking care to output the charge scalers via charge####.txt.

Data Analysis

At this point, data analysis must be done by a BCM expert (Dave Mack).

The basic principle of the analysis is as follows. The Unser monitor has a well known calibration, but it has an offset that is both noisy and drifts with time. In taking the calibration data, each different beam current is bracketed by a period of beam off so that the drifting offset can be well determined for each different beam current used in the cabibration. With these offsets determined, the average current for each nominal current is well determined and the unser calibration can be trasnferred to the BCM's. The BCM's are not linearly, particularly at small currents, so the calibration fit is made using only currents over the range that is required by the experiment. Typically this means that zero current is excluded from the fit.