[Brahms-l] Background in RHIC

There has been significant discussion in the last weeks concerning the
background rates seen at
Phobos at the higher intensities. The machine people is investigating this,
and willcontnue to do so over the weekend. this will imply lower collisions
rate (10-20%) due to running with lower number of bunches
and will also imply a longer 'defualt running' with the stores.

Some information is given below (CA-D+phobos reply)


----------------------------------------------------------------
Flemming Videbaek
Physics Department
Brookhaven National Laboratory


Wolfram Fischer wrote:

Dear experimenters,

the PHOBOS background is currently limiting the RHIC luminosity.

(1) Neither orbit bumps in IR10 nor the alignment of the abort gaps
at PHOBOS has eliminated the problem.

(2) The current store with only 45x45 bunches has no background
problem, but delivers 20% less luminosity than a store with 56x56
bunches.

(3) We will test another store with 45x45 bunches to see if thre
pressure rise is suppressed consistently. The orbit bump will
be taken out. Over the weekend, we propose to increase the bunch
intensity and bunch number to the point where the PHOBOS background
reappears.

(4) It is very likely that the consistent suppression of the PHOBOS
background can only be achieved by reducing the luminosity for all
experiments (by about 20% is my rough estimate). Alternatively,
PHOBOS could accept the high background at the beginning of each
store. On Monday we may need to find a compromise, please be prepared
to state your preferences.

(5) PHOBOS has repeatedly raised concerns about the increased dose
coming from the pressure rise. We would like to request an quantitative
analysis of this effect. Has silicon degradation been observed? If so
how does the degradation compare with the expected detector lifetime?
I have attached a file listing all fills with the PHOBOS pressure rise
problem, and the time the problem existed. By my accounts, we had a
total of 24 hours with high backgrounds.

Wolfram



-- 

Subject: Re: Run-4: PHOBOS background

Dear Wolfram,I would like to reply to point 5 below.
Peter Steinberg will address other issues at the Monday meeting.  Phobos has
two concerns about radiation:1  Chronic dose to detector and chip silicon  -
degradation expected around 15-20 krad.  Estimated dose so far is ~4krad,
and no degradation has been seen.  Rate of accumulation has roughly doubled
or tripled since vacuum excursions began.  We are NOT concerned about
changes in this rate, since the higher numbers roughly correspond to our
(conservative) pre-run guess of expected chronic dose rate.2.  Acute dose
rate to amplifier chips mounted at detector -  We have regular occurrences
of latch-ups of chips due to ionization at the junction.  We have shutoff
circuits that can protect against a few channels latching, but we cannot
safely run with bias voltage ON above an instantaneous dose rate of 1mrad/s.
This acute dose rate limit is the reason we cannot run during the very high
vacuum excursions.It should also be noted that backgrounds during the very
high part of the excursion become the dominant source of triggers, but I
will leave to Peter to address this quantitatively. -Don
Don BartonControls Division, Collider-Accelerator Department
Bldg 911c,  Brookhaven National LaboratoryPO Box 5000Upton, NY  11973-5000
Phone: (631) 344-7925Fax: (631) 344-5443Sec'y: (631) 344-2625 (Penny)


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