Nov 2013

Nov 7, 2013

Catching up ...

Over the summer, a student Joshuah Hill did a SULI project. He measured the asymmetries with FADC and compared them to the asymmetries measured in scalers. He did a lot of other low-level exercises first, however, to learn about the FADC, how to set it up and calibrate it, and he checked that the data makes sense. Since Kalyan have already done that in this logbook, I'll skip the low-level results and mainly state Joshuah's results. He found good agreement between FADC and scalers below a critical DAQ frequency f_c ~ 300 kHz where the deadtime (DT) is zero. Above f_c the DT increases rapidly and we could not correct the asymmetries. The value of f_c depends on the window width, this was for a width of 120 nsec. Here is a report I gave at a Compton meeting in September:

https://userweb.jlab.org/~rom/ComptonDAQ_26sep13.pdf

Next step:

We want to measure the deadtime in each helicity window. It seems the DT fluctuates and is mostly zero. The probability for DT to be nonzero depends on rate. For rates a bit above f_c (say, up to 400 kHz) there are a lot of helicity windows with zero DT, so that data should be salvagable. This is what I will attempt to do next.

Using a Caen 1495 with FPGA programming of inputs mapped onto outputs, I make a triple coincidence between helicity, busy signal, and a high-rate (3 MHz) pulser. I do this for both plus and minus helicity. I am reading this scaler each event. Now we'll see if I can get stable asymmetries above f_c ....