In this section, the cross section, rate, required total number of events, and the estimated
beam time for reference cell runs during commissioning are presented. The beam energy is 1.198
GeV. At an angle of 20 the (reference) cell windows will be within acceptance and its contribution
is estimated using:
.
Elastic scattering from window has a cross section of
at 10atm,
which gives a rate of 22.1 kHz with beam current 15. For quasi-elastic window
gives
at 10atm. During commissioning prescale factors
should be set properly such that the high rate from window will not kill DAQ. In on-line
analysis windows need to be cut off (in ) to make sure if enough data has been
collected.
N elastic (Mott) cross section is
.
With beam current 15 and N pressure 1 atm the rate is 132KHz, which exceed the
upper limit of data aquisition system (2KHz). Because of the contribution from cell windows
the real rate from N is a little less than 2KHz. For measuring pressure curve we would like
to have 20K events for each point so the error will be at 1% level. So we need
approximately 20K/2K/2.5 atm=4 seconds for 2.5 atm point and even less for higher pressure.
Rate is not a problem but need to pay attention to the prescale factors
such that the event rate is lower than the DAQ limit.
We would like to have false asymmetry
, e.g.,
events are needed. At elastic peak this corresponds to 25M/132KHz=3 minutes. With beam effiency 60%, also taking into account the fact that we are using Mott cross section to estimate rate, about 10 minutes (one run) is needed for N false asymmetry measurement.
He elastic cross section is . With beam current 15 and He pressure 1 atm the rate is 50Hz. The quasi-elastic peak will also partially fall into the spectrometer acceptance, the ratio of quasi-elastic events vs. elastic events is about 0.4. If windows contribute 22KHz then with prescale factor 12 the real rate coming from He elastic is about 4Hz/atm. For each pressure the quasi-elastic event rate, time needed for 10 K events and the total time (with beam efficiency 60% and an extra 5 minutes to change pressure) are shown in table B:
He pressure (atm) | rate(Hz) | time for 20K events | total time |
2.5 | 10 | 17 min | 17+5 min |
5.0 | 21 | 8 min | 8+5 min |
7.5 | 31 | 6 min | 6+5 min |
10.0 | 42 | 4 min | 4+5 min |
He elastic cross section is
.
With beam current 15 and He pressure 10 atm the rate is 800Hz. With a prescale factor of 12
due to window contribution the real rate coming from He elastic is 66.7 Hz/atm.
To obtain 20 K events about 5 minutes will be needed. So we will need 9 minutes (with efficiency 60%)
for this measurement. Events within elastic peak should be no less than 20K.
C elastic cross section is
.
Event rate will be 60Hz with beam current 5, 108 mg/cm C target (density 2 g/cm,
thickness 0.5mm) and sieve slits in (which will reduce the acceptance by 10). To obtain 10 K events
about 3 minutes will be needed. So we will need 5 minutes (with efficiency 60%) for this measurement.
C quasi-elastic cross section is
.
With beam current 5, 108 mg/cm C target and sieve slits in the rate is 1.2 KHz. We
would like to have 5 minutes on this measurement.
The cross section is shown above. Without collimater the rate is 120 KHz. To check asymmetry at a
level of
we need 2.5 events so we need 25M/120K = 3.5 minutes, or total
beam time 6 minutes (with efficiency 60%) for this measurement.
elastic asymmetry, with target spin parallel to beam, is estimated to be 0.0877.
With dilusion factor ,
beam polarization 80% and target polarization 40% the raw asymmetry is
.
If an error of 3% is needed then the total number of events will be
. With beam current 15 ,
typical
cell pressure 10 atm, prescale factor 12,
elastic rate is 40 Hz.
Contribution from quasi-elastic scattering is about 0.4 times elastic rate. With both arms taking data
simutaneouly we need effective beam time
,
or total time
hours for
asymmetry measurement. Number of events within
elastic peak is no less than 2.3M. Total number of events from both elastic and quasi-elastic is 3.22 M.
Since rate of elastic scattering from windows is high compared to the rate coming from elastic, the first run for asymmetry will be analyzed on-line so as to check if it is possible to suppress window contribution by shifting the central momentum of HRSs lower. By doing this quasi-elastic scattering from windows still come into acceptance and elastic will also be suppressed by a factor of 1.33.
Momentum setting for resonance at 1.198 GeV and 20 is 0.796 GeV/c. Total cross section
(by QFS) is 8.693b/GeV-sr, contribution from resonance is 6.121b/GeV-sr. With beam current
5 , typical
cell pressure 10 atm, total rate is 6.541 KHz (4.605 KHz from
resonance), which exceeds DAQ limit. Also take into consideration windows, we assume a prescale factor of 12
is needed. Then the rate is about 2KHz/12=160 Hz.
resonance asymmetry is about 0.02. With dilusion factor ,
beam polarization 80% and target polarization 40% the raw asymmetry is
.
If we need to measure asymmetry away from zero to determine sign, then the total number of events will be
. With
rate 160 Hz,
one arm taking data, we need effective beam time
,
or total time
hours for
asymmetry measurement.
Number of events within resonance peak is no less than 120K.