Policy for Experiments with the Microball

It is to your advantage as a potential Microball user to pay attention to the following restrictions and recommendations that apply when a 4p charged particle array, like the Microball, aims to accommodate a variety of experiments with diverse requirements.

Problems in Running with the Microball - Some Advice

It is well known that no charged-particle detector can survive long if it is hit by elastic scattering. The solution to this problem is to place protective absorbers in front of each CsI(Tl) detector, which is sufficiently thick to stop the most energetic ions of each beam species and yet to be thin enough not to stop most of the protons or alpha particles.

Therefore:

1. As an absolute rule, no elastic scattering will be allowed to hit the CsI detectors.
2. There is a significant demand for a large variety of reactions and energies to be run with the Microball. Clearly it is not possible to select the optimal absorbers for each Experiment. We will need some understanding that compromises have to be made in an effort to please as many people as possible.
3. The factors that must be preserved are :
4. • Identify the channels as well as possible.
   • Measure particle energies as well as possible.
5. We must select the absorbers for a "typical reaction". This means that the absorbers for some reactions will be too thick and for some others too thin.
6. We have chosen "The typical reaction" to be: A + B = (Z=50, A=110)* with 5 emitted particles on the average. This establishes the Maximum Energy of Beams that Stop in the Present Microball absorbers. The current list is available here at the bottom of the parameters file.
7. Since at the end of each scheduled group of experiments there is some radioactivity deposited on the absorbers of the first ring of detectors, we have often replaced these. On occasion we have changed absorbers somewhat from the standard set when it was deemed that this would improve the data quality for the group of experiments. So make the your calculation as to what the optimal absorber thickness inside the grazing angle is for your reaction and let D. Sarantites (dgs mail address below) know about this well in advance of the scheduling of your experiment.

Restrictions in Running the Microball

We note the fact that 10¹³ of 50 MeV ¹²C ions kill the CsI scintillator resolution [see Miersch et al. NIM A369 (1996) 277] with Heavier and more Energetic Ions being worse.

FOR THIS REASON, THE PROCEDURE FOR RUNNING THE MICROBALL MUST BE:

1. Limit the Maximum Rate of any type of particle to < 5,000 c/s per detector . This is already too high for a good performance of the Microball, but it is also high enough to keep up with nearly the maximum rates of Gammasphere. Pay considerable attention to the way the Gammasphere dead-time is determined and what fraction of that is comes from the Microball. Understanding this will allow you to do a better experiment, (get more "good" data).
2. Set the discriminator thresholds above noise and then look for Excessive Counting Rates in the most forward rings of the Microball. RAISING THE THRESHOLDS TO REDUCE THE APPARENT RATES WILL NOT BE PERMITTED.
3. Turning the power to the Microball OFF and RAISING THE BEAM INTENSITY ALSO WILL NOT BE PERMITTED.

Understanding the Absorber problem (?)

Normally one might expect that absorbers may be needed only inside the grazing angle. Experience with all particle detection experiments that run at high beam intensities is that protective absorbers are needed even for the back detectors. The alternative is a lot of noise from target electrons and/or X-rays that are piling up.

To reduce this problem we cover all the Microball detectors with absorbers which are thinner at the back angles. It turns out that the electrons/X-rays strongly increase with beam intensity and Z of the target and Z of the projectile. For high Z targets the situation is a lot better because the Coulomb barrier in general boosts the kinetic energies above the cutoff energy of the absorbers at the back angles. In general we try to use high Z absorbers because they are much more effective in stopping high Z ions relative to low Z ones. Au is the best, but we cannot afford it. Tantalum is next, but rolling it to the desired thickness is very hard. Lead is also good for the thick absorbers, but there are large non-uniformities in rolled lead. Tin-lead allow is only available as 5.0 mg/cm² and this is not always convenient. This will tell you why in the list of absorbers you will find a variety of metals used.

We never have loss of particles in the front angles for any projectile-target combination. This is not true at the back angles. There, the loss of particles, mainly a's, becomes worse with decreasing Z of the target. One the other hand, because the target Z is lower the number of electrons/X-rays is smaller. It is not clear that anyone understands the quantitative solution to this problem.

Another situation in which loss of efficiency and performance appears is in reverse kinematics reactions. As the Z_proj/Z_target approaches and surpasses 1.0 one has to consider each case carefully in estimating expected losses. We will not solve this problem here.

For further Information contact:  dgs@wuchem.wustl.edu.

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Last update: Wednesday, 19 November 1997; 21:49.