# Presentation: 013102

## RHIC Collimation

- Ray Fliller
- and
-  Angelika Drees

- RHIC Retreat '03

## Outline

- Effect on Experiment backgrounds.
- Crystal Collimation
- Plan for the New Collimation System

- This talk IS about:

- This talk IS NOT about:

- Why these backgrounds hurt the experiments
-  Vacuum related backgrounds
- Gap Cleaning
-  Anything else we have heard this morning

## Collimator Layout

- During operations, the crystal collimator was not used (more
- on this later!)

## Problems we could "fix"

-  PHENIX MuID system background
-  BRAHMS background during dAu when PHOBOS *=4m
-  STAR yellow halo background (and caused some!)
-  STAR blue halo background

- Problems we couldn't...

-  PHOBOS backgrounds (Ubaldo's talk...)
-  Pressure rise around blue collimator

## Collimator Operational Issues

-  Collimators needed to be inserted "by hand".  Operators don't like moving collimators around out of fear of killing a store.  Sometimes backgrounds where not minimized.
-  Steering at PHENIX requires collimator retraction.  "Lost time" after steering due to high backgrounds.  Also affect vernier scans.
-  Sometimes STAR and PHENIX were coupled. STAR and PHENIX had to decide who would live with some extra background. Reducing backgrounds at PHENIX could raise STAR backgrounds.

## Collimator Performance - dAu Run

- Reduction

- Optics

- BRAHMS

-  STAR Blue

- STAR Yellow

- PHOBOS 4m

- 2-3 x

- 0 x

- 2-3 x

- PHOBOS/ BRAHMS 3m

- <2 x (usually 0)

- 4 x

- 0 x

- Reduction

- Optics

- PHENIX Blue

- PHENIX Yellow

- PHOBOS 4m

- 0 x

- 5 x

- PHOBOS/ BRAHMS 3m

- 9 x

- 9 x

- During initial Physics running (all IR's at 2m) collimators were not really
- used, but collimating PHENIX sometimes increased STAR's background.
- PHOBOS never benefited from collimation.
- BRAHMS was affected by Yellow horizontal mostly .
- PHENIX benefitted from vertical collimation (both rings).
- STAR blue halo reduction was from blue horizontal.

## Collimator Performance - pp Run

- Reduction

- Optics

- PHENIX Blue

- PHENIX Yellow

- all IRs

- 0 x

- 3 x

- STAR & PHENIX

- 8 x

- >10 x

- Reduction

- Optics

- STAR Blue

- STAR Yellow

- all IRs

- 1.5 to 10 x

- 1.5 to –2 x

- STAR & PHENIX

- 5 x

- 1.5 to –1.1 x

- PHOBOS and BRAHMS saw no effects of collimation.
- Collimator preformance independent of rotator configuration.
- Yellow collimator could increase STAR background!
- Blue collimator was always helpful.

## Collimator Pressure Rise

- Store 3244, Pressure in Blue collimator section increases as soon as the collimator touches the beam.  It was never enough to trip the machine,
- but large enough to worry.  Does not happen all of the time (next store rise not seen), but did happen with d and p.  Never seen in yellow.

- Collimators touch beam (PIN Diodes go up ~1.6 kHz)

- Pressure

- Collimator Position

- PIN Diodes

## Crystal Collimation

## ...Means rotating it!

- Because of the tilt of the ellipse

- Moving the crystal...

- And the angular spread on the crystal
- is too big!

## New Collimation System

## Collimator Issues This Fixes (we hope!)

- Sometimes STAR and PHENIX were coupled. It is hoped that the secondary collimators will reduce "background coupling" between experiments by reducing secondary halo.
- Collimators needed to be inserted "by hand". An system is planned to use PIN Diodes, BLM's, backgrounds and DCCT to insert collimators automatically.  Sounds scary but The Tevatron already uses such a system, and they are VERY happy with it.
- Steering at PHENIX requires collimator retraction.  Because the collimators are planned to be in the same area, this is still an issue.
- Collimator Pressure Rises. Whole new ball game.

## Conclusions

- The collimators were helpful during the dAu and pp runs for reducing backgrounds at most of the experiments.
- The crystal collimator worked, but not well enough
- New collimators will be installed in the 8 o'clock warm straights downstream on PHENIX