Hi Vitaliy,
regarding your Q2 distribution:
-the fact that with the exception of the lowest Q2 bin, all other bins
show ca. 20% more yield for the e-left-n-right combination than for the
opposite combination can be explained by the fact that on the left sector
one neutron bar isn't counting (elog 16526) (LNL2) and another one is
counting low compared to the remaining six bars, which corresponds to 75% relative
efficiency compared to the right-sector Ohio wall.
-the higher yield at Q2=0.05 for the e-right-n-left combination may be
explained with lower detection threshold for the left-sector wall.
Q2=0.05 corresponds to a neutron energy of 25MeV, see also John Calarco's
email to [BLASTTALK] from 9/24.
Indeed, scaler rates are higher in the left sector, and also the average
high voltage supplied to the phototubes is higher on the left sector,
whereas the hardware thresholds are all set to 20mV for both left and
right.
In the time-of-flight spectrum for the left sector neutron wall, the
distribution for the neutrons is more extending to larger times as for
the right sector, corresponding to smaller energies, corresponding to lower Q2.
regarding the "different photon yield mystery": why should one expect the
same yield for e'gamma events regardless wether the target is empty or
not? Many (if not most) photons have their origin from the decay of
pi_0's, for which the photoproduction cross section scales with the target
mass number. This explains the excess ratio of e'gamma events from D2 to H2 of about 2:1
when the contribution from the empty target is subtracted.
Very nice first analysis!
Michael
On Fri, 3 Oct 2003, vitaliy ziskin wrote:
> I was looking a the Ohio wall performance during the run with the
> collimator in for three different targets, D2, H2 and empty. All plots
> are normalized to beam charge.
> Background:
> The first plot is the plot of the reconstructed vertex for what I call
> "en" events, that is electron on the left/right (top/bottom) and a
> neutron counter hit on the right/left with not tof scintilator hit.
> Already data shows a rise above background for D2 target, especialy for
> the right neutron wall (top plot).
> The second plot is the plot of a neutron wall time of flight for both
> sides. The what we call "gamma" peak is clearly visible for all gasses,
> however the second peak is only present for D2 gas. It is still a
> mistery why there is less "gammas" for the background events. This
> could be due to the misidentifying "fast" neutrons as gammas, because of
> the relatively wide "gamma" peak.
> The third plot is the reconstructed vertex witht he cut on what I call
> "neutron" peak (or second bump). The situation becomes better. The is
> still much more background in the left Ohio wall than the right. I
> think these are charge particle that hit the neutron wall from the other
> side (bend in the ring) and never reach scintilators in coincidence with
> low energy electrons in the other sector.
> This is what I did next, cut out low energy "crap" (<0.2 GeV). The next
> plot is the vertex for this cut. The background is now comparable in
> both sectors at about 6%, which is managable.
>
> Rates:
> The next plot is the Q^2 plot for the "good" e-n events for both
> sectors. The count rate is normalized to charge.
> Since I cannot reconstruct the missing mass right now for the next plot
> I cut on the energy transfer <0.45GeV to isolate quasi-elastic events.
> The last plot shows my best guess for the quasi-elastic events. I
> cannot explain the difference in rate for each sector. For now I will
> use electron-left, neutron-right data.
> Assuming an average current of 80 mA or 12.5 seconds per coulomb I
> estimate the following rates for this flow (0.1 sccm):
>
> Q^2 rate/coulomb rate/sec events in
> 800 hours
> 0.05 0.2
> 0.016 46080
> 0.15 0.52
> 0.0416 119808
> 0.25 0.21
> 0.0168 48384
> 0.35 0.08 0.0064
> 18432
> 0.45 0.03 0.0024
> 6912
>
> The rate at Q^2 of 0.25 is comparable to Igor's value if scaled by the
> correct flow (~0.07 scmm) and corrected for higher energy ( I have not
> done this yet). So, in order to do better than NIKHEF we need to
> increase neutron detection efficiency and do better with the figure of
> merrit (target polarization). Also, we can try to increase average
> current.
>
> Best regards, Vitaliy
>
>
--+-------------------------------------+--------------------------+ | Office: | Home: | |-------------------------------------|--------------------------| | Dr. Michael Kohl | Michael Kohl | | Laboratory for Nuclear Science | 5 Ibbetson Street | | MIT-Bates Linear Accelerator Center | Somerville, MA 02143 | | Middleton, MA 01949 | U.S.A. | | U.S.A. | | | - - - - - - - - - - - - | - - - - - - - - -| | Email: kohlm@mit.edu | K.Michael.Kohl@gmx.de | | Work: +1-617-253-9207 | Home: +1-617-629-3147 | | Fax: +1-617-253-9599 | Mobile: +1-978-580-4190 | | http://blast.lns.mit.edu | | +-------------------------------------+--------------------------+
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