Minutes of the (extended) Meeting Tuesday, June 25th 2002   Keeper of the Minutes: Sabine Engel   Present (in order of appearance):  SE, DAH, MC, JR, PJ, CW, ML, JF, LB, DO, DH, AC, AO, DG, SB   Last minutes: Declared adopted after the correction of one typo in the date of the previous meeting. It has to be June 4th 2002.   Light Entertainment: Leaky beam studies performed at the previous week-end (21.-23.06.) were presented by DH, JF and DAH. DH: looking at the runs from May, it could not be concluded, at which point Q10 was changed. DAH compared the rates for leakies in 20Ne(p,g) at 360 keV/u for different target vacuum conditions. It could be shown, that the rate went up by  more than 50%, when the downstream pressure was increased by shutting off turbo pump TP21. The rate went down for decreasing central pressure. JF presented the results of a study to find the path of the leaky beam in the first half of DRAGON. Since the leakies seemed to appear at the negative side of the mass x slits, which would correspond to an inverse magnification from the positive side of the charge slits, the mass slits were set to a negative point in order to center the leakies, while the Q-slits were moved in three steps towards positive center points. However the leaky beam rate did not increase. Therefore no conclusions could be drawn concerning their origin.   Heavy Entertainment (no weapons involved): SE presented a plot of BGO efficiency versus beam energy for the resonance at E_cm=1113 keV in 20Ne(p,g). The expected gammas of 3.5 MeV result from a 91% ground state transition. Nevertheless the coin/sgl rate was in the order of 27%, much less then expected from simulation. DG simulated BGO versus position in the target and showed a plot for 4 MeV gammas. From 45% central value the efficiency fell off to 40% for +/- 5 cm distance from the center and further to 25% at +/-10cm. The curve showed two dips, where the beam line is shielded with lead and an upstream/downstream asymmetry for the same reason. SE: The transmission through the pumping tubes measured with the wobbler studies is less than expected. >From a central plateau it drops to 90% transmission for +/- 13.8 mrad (in x and y) and 50% for +/-17.3 mrad (x)  or 17.4 mrad (y). Since the maximum opening half angle for the higher resonance is only 7.5 mrad, we can assume 100% transmission, while for the lower resonance some systematic errors have to be taken into account. The cuts used to determine recoils from leaky beam in the analysis of the 822 resonance in 21Na(p,g) were shown. Those cuts applied to the coincidence events cause a loss of 4% of the counts over all runs. In order to compare two different ways of beam normalization, SE compared the ^Óelastics method^Ô with the ^Óbeta monitor principle^Ô, which seem to agree for most of the runs. However since the beta monitor is independent of beam energy, a general relation between beta monitor rate and FC4 can be achieved for all runs, while with the elastic monitors each energy point requires a new normalization. Therefore the beta method is to be prefered. Looking closer at our favorite run #6214, the question is whether to use the same cuts as for the other runs, or to tighten them more due to high leaky beam background. If the same cuts are applied as for the other cases, the point lines up with the neighboring points on the fit. The resonance was fitted again leaving the target pressure as a parameter adjusted for each run. With the counts normalized to the beta monitor, and the errors in energy projected to the error in yield, the result gives wg = (491 +/- 50) meV, Gp = (17. 9 +/- 3) keV, Er = (821.2 +/- 1) keV, with an chi^2 of 1.61. DAH showed his fit of the elastically scattered protons that would lead to similar results: Gp = (20.2 +/- 1.6) keV and Er = (823.1 +/- 0.3) keV. (all units in center of mass frame)   The fit, the art of fitting and the preferred way to present the results were discussed intensively among the group.   JR raised the question on how well the beam energy is determined. It was concluded, that a 0.2-0.3 % uncertainty for each measurement should be assumed, since the beam position is not confined at the entrance to the gas target which could lead to slightly different centroid positions. Using the time-of-flight from the accelerator to the BGO does not lead to more reliable results either, since temperature in the cable, phase shifts of the ISAC tune and other unknowns already cause a spread in JR tof-vs-MD1-plot.   At 12.15 pm the DRAGON group was ready for a break. The meeting was continued at 1 pm.   JR presented the figures he intends to show on his poster at the NIC conference: DRAGON optics, target and electronics, two thick target yield curves of the 15N(p,ag) resonance measured last summer at different energies and some plots imaging the beam suppression of the DRAGON in the case of 21Na(p,g) at 212keV.   SB guided the group through his data analysis. For the coincidence events a tof cut as well as a gamma energy cut at 3 MeV are done. To normalize the yield to beam on target SB used the elastics energy spectrum, where he fitted the high-energy background and extrapolated the fit to the low energy, where he subtracted the background from the proton peak. Since this method might lead to a small error at the low energy tail of the protons, SB used the elastic time spectrum as a second source. Here the background shows no rf-correlation and can be distinguished easily. However, for the April runs the elastic monitor does not give a reliable time signal. SB will look into using the beta monitor. Presenting the yield curve the question arose, whether all of the points originally taken as a thick target measurement could still be regarded as such. With a target thickness of roughly 11 keV/u, both the runs at 205 (??) and at 215 (??) could lie on the edge. SB will scale the points for each energy with the individual BGO efficiency provided by DG and compare the yield versus energy again. JR suggested therefore to use the points in question only to determine the edge and the resonance energy, while the yield should be calculated with the runs we definitely know are in the target center. AO pointed out that otherwise what we gain in statistics might be destroyed by additional systematic uncertainties that would be introduced.   Still an open question remains why we seem to agree with the resonance energy published by Claus Rolfs et al. for the 822 resonance but not for the lower one. Nick Bateman based his energy calibration on Rolfs, but collaborators at Yale working on a transfer reaction seem to reproduce Rolfs energy published as 212 keV in cm.   Contrary to earlier hopes it was agreed that the results on the 212 keV resonance will remain ^Ópreliminary^Ô, maybe with a line to ^Óguide the eye^Ô.   SE suggested to present: the transmission through the pumping tubes, the beam energy calibration, target density distribution, cuts used to determine recoils in the 822 resonance and the yield curve of the 822 (normalized to beta). If the measured BGO efficiency will be presented together with a simulation depends on whether or not the discrepancy in total efficiency can be resolved. SE will look into the problem.   The results of the measurement of the 790 keV resonance in 24Mg(p,g) cannot be used to determine the overall efficiency of the DRAGON, since the published values differ by significant amounts. However SE suggested publishing the DRAGON data as an independent measurement.   SE also plotted the rate of elastics versus beam energy for the 822 keV resonance and obtained slightly different results than DAH. Both will compare their data.   LB stated the fact that DRAGON results do not agree with TUDA, presenting a yield curve from a small angle TUDA detector for the same resonance. LB ruled out the theory that an interference with another resonance could cause the discrepancy.   LB will give an invited talk at NIC, giving an overview over ISAC; TUDA and DRAGON.   The meeting was finally closed at 3.30pm, holding the record of the longest meeting by far in DRAGON history. [ Part 2, Image/JPEG 175KB. ] [ Unable to print this part. ] [ Part 3, Image/JPEG 127KB. ] [ Unable to print this part. ] [ Part 4, Image/JPEG 269KB. ] [ Unable to print this part. ] [ Part 5, Image/JPEG 108KB. ] [ Unable to print this part. ] [ Part 6, Image/JPEG 189KB. ] [ Unable to print this part. ] [ Part 7, Image/JPEG 117KB. ] [ Unable to print this part. ] [ Part 8, Image/JPEG 141KB. ] [ Unable to print this part. ] [ Part 9, Image/JPEG 85KB. ] [ Unable to print this part. ] [ Part 10, Application/MSWORD 40KB. ] [ Unable to print this part. ]