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            Minutes of the Meeting of 05. March 2002, prepared by SE

Present:

Dave Ottewell, Dave Hutcheon, Art Olin, Lothar Buchmann, Cybele Jewett,
Alan Chen, Joel Rogers, Mohan Chatterjee, Peter Machule, Dario Gigliotti,
Shawn Bishop, Sabine Engel

 

Agenda:

1.      last week's  minutes

2.      business arising

3.      hardware status

4.      offline analysis of 21Na(p,g) at 822 keV

5.      outlook on next week

 

1.      minutes:

	No errors or omissions.

 

2.      business arising (DAH):

The beam instability tests at various energies, pressures and beams
included runs with fading 21Na beam where the low count rate led to
errors in the TDC. The actual beam instabilities for runs with reasonable
parameters might therefore not be as bad as reported by CJ last week.

 

3.      hardware status:

DAH reported:  The fence around DRAGON is almost mounted, except for
gates and locks at various entries.

AO: The EPICS scaling routine works, but is to be treated under the
assumption ^'garbage in, garbage out'. A more user-friendly set up to
reduce the 'garbage in' aspect would be desirable. Also EPICS could be
further improved by setting the font size to reasonable values and
allowing printing of pages.

DAH: Having control of EPICS again Miguel Olivo started re-commissioning
the 2 electric dipoles ED1 & 2. So far the cathode has been raised to 190
kV in each tank.

PM: The beam centering devices were checked. A broken connection on BCM2
was fixed. Unusual behavior of BCM1 and 6 during the last run time could
not be related to the hardware. To rule out software problems they will
be tested with a current source.

PM & DAH: The thermocouple connection needs to be fixed.  Will try a
repair rather than complete replacement.

JR:  The lead shield of the BGOs should be improved for easier mounting
and dismounting.  Not a high priority item.  Hart should look at it.

 
4.      offline analysis of 21Na(p,g) at 822 keV (AC & SE):

SE presented the results so far on the analysis of the 21Na(p,g) reaction
at the higher energy of 822 keV. 12 'good' runs were identified and
analyzed with NOVA. The energy of each run was calculated from MD1 behind
the gas and scaled to the incoming energy using SRIM. Only recoils from
the singles bank were used and constrained by fairly wide cuts on rf-time
(~55ns), position and energy. The charge state distribution was
determined from the recoils of 4 consecutive runs at the same energy, but
different DRAGON tunes. Corrections for dead time were made, as well as
for singles loss due to double hits on the backstrips of the dsssd. JR
remarked that the efficiency of the backstrips might be different than
for the front strips, but the counts in front and back strips were
monitored for each run and did seem to be equal. The way the count rate
in the elastic monitors should be related to incoming ion beam was widely
discussed and will be further looked into. NO CORRECTION FOR BEAM
TRANSMISSION THROUGH THE SEPARATOR WAS DONE YET.

All errors were calculated using the statistical uncertainties in the
recoil and elastic rate, leading to total errors of approximately 8%
errors in the yield for most of the points, but 30% for the almost-off
resonance measurements. The error in the beam energy includes 0.2%
uncertainty in the method of determining it from MD1, however
UNCERTAINTIES IN THE ENERGY LOSS (SRIM) ARE NOT INCLUDED but would have a
direct impact on the resonance energy.

The yield curve was then fitted with a convolution of a Breit-Wigner
function and an energy loss profile, which was simulated by a double
arctan function, basically a rectangular function, assuming a constant
detection efficiency over the length of the target. The total energy loss
in the target was treated as a free parameter. However the result led to
an unreasonable energy loss and has to be further investigated.

A vivid and interactive group discussion arose on the subject of how the
target profile can, should or should not at all influence the yield
curve. LB pointed out that further challenges might be caused by the
fact that TUDA data indicates a so-far-unknown resonance just below the
one observed.

Due to high 511 background only the coincidence data can be used to
estimate gamma branching ratios. Here low statistics hamper a proper line
fitting. So far only rough estimates are available. Two stronger peaks
were observed related to the gammas emission to the ground and the first
excited state at 1.246 MeV. A weak third line in the 3 MeV range can be
observed. It could either be related to a transition to the second
excited state at 3.308 MeV or as the second part of a cascade to a state
at 4.4 MeV (first gamma below energy threshold) and from there to the
first excited state. More analysis is needed.

During the upcoming 21Na beam time in April we need to measure the energy
loss in the target. Also one or two more points on  the yield curve would
be helpful.

 

5.      outlook on next week:

DL will be asked to report on the Wobbler studies.

SB is preparing summary of the latest results on the analysis of the low
energy 21Na(p,g) resonance at 212 keV, including details on the treatment
of low statistics phenomena.

The new minutes ball is in JR's court.