[BTeam] Fw: [EXTERNAL] Follow up on Hall B Halo Rates Troubleshooting

[Sebastian Kuhn]

Dear all,

since this question keeps coming up: When we “switch the target”, we simply fill the SAME (unmoved!) structure with a different gas. There is no way that this can change anything about the relative position/orientation of beam vs. target. We have been doing this many times since the beginning of the summer run (going from D2 to H2 to “empty” to 4He and starting with D2 again), so the fact that ONE of those changes happened somewhat close to the change in backgrounds is purely accidental. (We also went through several target changes since the background increased, with NO change, including running with “empty” target again). Finally, the density (in terms of g/cm^2 and in terms of radiation lengths) of the actual gas target is miniscule, whether it’s empty, filled with D2 or filled with 4He. The only more massive structure where some background COULD come from is the beam line exit window area in front of the target. I can’t think of any way how this area could have changed over time, but consider it much more likely that it simply gets sprayed by whatever is generated upstream of it.

More generally, there are ample indications by now that the background, whatever it is, MUST be coming from upstream of the target (including the fact that the BOM, which sits upstream of the target, sees more than double the rate per nA).

Thanks for thinking through all possibilities — I’m hopeful we’ll find the culprit soon! - Sebastian

We have a question raised:
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interesting find by dennis. elog 3835820 .
on that day where we got higher rates upon restoring machine, hall b switched from empty hydrpgen ( which i assume is just to get the backgroumd rates for cell walls ) to helium 3.

i am curious as of what is the diff. in radiation lenghts abd should they expect higher rates ?

 it would be useful for Mike T. to clarify this with Sebastian Kuhn.  it might be irrelevant or maybe not ?
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It seemed worth asking.

Michael
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