# Presentation: 202725

## The RHIC Absolute Polarimeter:CNI + Polarized Gas Jet-TargetAlessandro Bravar et al. RHIC Spin Coll. Meeting

-   Introduction
-   Elastic pp kinematics
-   Setup
-   Backgrounds
-   MonteCarlo Studies
-   Recoil detector

## Method

- Choose a process with large  and known AN
* **pp* elastic scattering in CNI region
- Current knowledge on AN (exp & theo) to poor for
- Pbeam / Pbeam < 0.05
- Measure AN to required accuracy of AN < 10-3 with unpolarized beam and polarized target Ptarg / Ptarg < 0.02)
- 	then measure Pbeam using AN with polarized beam and unpolarized target
- Pbeam / Pbeam ~ ****2 AN / AN   (some systematics counts twice)
- or Transfer target polarization to beam polarization:
- 	i.e. Measure ratio of spin asymmetries with beam and target polarized
- Pbeam / Pbeam ~ AN / AN   (some systematics cancels)
- NB *Self-Calibration* works with elastic scattering only

## Elastic pp   d / dt   &   AN

- 0.001  t  0.02 GeV2
-  = 3 mbarn
- AN  0.03

- E704@FNAL

- UA6@CERN

- t range

## RATES

**L** =2 1011 **** 120 **** 78 103 **** 3 1011 = 5.6 1029 cm-2s-1
-                                                        = 4.7 1027 cm-2s-1 / bunch
- N = **L** ******** *acc *(****/2) *eff* (50%) = 70 evt s-1
- 							    ~ 0.5 evt s-1 / bunch
- in 12 hours can collect 3 x 106 events

| BEAM | TARGET |
| --- | --- |
| 21011 p / bunch
120 bunches
78 kHz | 
3  1011 atoms / cm2 |

## Kinematics

- R; ER

- S

- Essentially only 1 free parameter: t (+ ) 
- 	       elastic *pp* kinematics fully constrained by recoil proton !

-          measure position and energy of recoil 

- R and *t*

-      t : 0.001 – 0.02 GeV2
- R : 1 – 5 degrees
- Tkin : 0.5 – 10 MeV
-   pR : 30 – 140 MeV

## Setup

-         energy ** ***t***
-         position  R
-         time

## Setup: top view                side view

## MonteCarlo Studies

-  Inputs:
-  *pp* elastic cross section (differential)
-  jet target profile (T. Wise)
-  target holding magnetic field (W. Meng)
-  Outputs:
-  design of recoil spectrometer
-  resolution of recoil spetrometer MX and R
-  separation of elastic *pp* from non-elastic *pp* (background)

## Jet-target holding field

- r (cm)

- Bz (Gauss)

-  Bd*l* ~ 0

- almost no effect on recoil proton trajectories
- 	left – right hit profiles almost identical
- 	left – right acceptance almost equal

## Recoil hit profiles @ 100 cm

- mm

- 65 mm

- 1o

- 4o

- 0.0015 < |t| < 0.015 GeV2

- 0.001 < |t| < 0.02

- diff. x-sect.
- kinematics
- target profile

## Background: We can live with it !

- Mainly three sources of background:
- Beam – gas interactions: at 10-9 torr, ~ 107 atoms / cm3
- Proton dissociation *pp **** **Xp*
- Beam – residual jet-target gas interactions (suppress with collimator)
- Acceptable level of background:
- Otherwise Background Subtraction

- For   ANBG ~ 10% (0%) ,    *R* ~ 1% (3%) acceptable

- For   *R* ~ 5% (2%) *R* ~ 5% (2%)   OK for ANBG ~ 30%
- For   *R* ~ 2% *R* ~ 10%          OK for ANBG ~ 30%

## |t| vs pos. from UA6  background

- two sources of background:
- a) beam gas interactions
-      few %
- b) beam proton dissociation
-     few % below elastic peak

- |***t***| ~ 0.008

- less than 5 % below elastic peak
- using the recoil technique only

- elastic *pp*

## pp Xp Dissociation

- Kinematically quite different from elastic process

- Suppresion of *pp* *Xp*  depends
- on how well one can reconstruct M*X*

- resolution on R depends on
- 	distance from target and
- 	target profile
- NB at threshold coplanarity doesn’t help !

- M2X (GeV2)

- proton

- (p* *+ )
- threshold

- MX

- M2X

## resolution

- MX and R depend on
- jet-target profile

- 1 free parameter to choose:
- distance from jet-target
- (could also narrow target profile)

- mm

- jet-target profile

- M2X (GeV2)

- R (mrad)

- ~ 3mm *core*

*wings*

- angular resolution

-   Dist.
- 200 cm
- 100 cm
-   50 cm

- (p* *+ )
- threshold

## time of flight

- interacting bunch

- following bunch

- ns

- arrival time of recoil protons at 100 cm

- |***t***| = 0.0015 GeV2

- |***t***| = 0.001 GeV2

- CNI peak

##  / dM2X

-  (*elastic*) ~ 100 x  (*pp*  *Xp*) at threshold !

*X* = proton

- (p + ) threshold

- @FNAL

- @FNAL

## t vs R

*t* vs R

- R (mrad)

- |t| (GeV2)

- Kinematically
- forbidden     allowed

*elastic*
-      (p + )* *[threshold !]
- 		      N(1440)

- reconstructed from:
- 1.   deposited energy
- 2.   hit position
- recoil spectrometer
- resolutions:
- R= targ. ext. / dist.
-         ~ 3 mrad
**|*****t*****|** = 2 *m**p* Tkin
- Tkin < 100 keV

## Si Recoil Detector

- Requirements
-  good energy resolution
- E < 0.1 MeV
-  space resolution
- x ~ few mm
-  time resolution
- t ~ 1 (2) ns

- horizontal segmentation ~ 4 mm (16 ch.)
- vertical segmentation ~ 8 mm (8 ch.)   ~ 10 mrad
- thickness: 0.5 mm  stops up to 8 MeV protons
- 	  veto for faster protons?
- under development (Inst. Div. @ BNL)

## Energy deposited in Si detector

- MeV

- |***t***| = 0.015 GeV2

- |***t***| = 0.02 GeV2

- mip in 0.5 mm of Si deposits 150 keV
- lowest energy protons (t = 0.001 GeV2, Tkin = 0.5 MeV)
- 			~ 3 mip signal
- signal / noise comfortable

## Readout

- 2 x 144 channels
- Similar to RHIC pC polarimeters
- preamplifier boards just outside of vacuum chamber
- signal transport to counting room with coaxial cables or
- 	optical fibers
- shapers (amplifier boards)
- Wave Form Digitizers (Satish @ Yale)
- Low Rate
- can record waveform for each event
- Under development (Inst. Div. @ BNL)

## Summary

- Comfortable statistics (3 **** 106 events in ~ 12 hours)
- pbeam / pbeam < 5%
- AN < 10-3 ... feasible, but ptarg / ptarg ~ 2% & bkg < few %
- Background under control (not too bad !)
- Roman pots not necessary in CNI region
- 		      useful for background studies
- Recoil detector based on existing technology
- Readout similar to RHIC pC polarimeters
- Recoil detector and readout electronics under development, first prototypes this fall
- Additional MonteCarlo studies under way
- Asymmetry Extraction under study