1. Polarized Gas Target for LHCb
Erhard Steffens (Erlangen)
for the target study group
Paolo Lenisa (Ferrara), Alexander Nass (FZ Jülich), Pasquale Di Nezza (Frascati), Frank Rathmann (FZ Jülich), E. St.
Introduction
Storage cell principle and development
Storage cell targets at HERA-e and COSY
Polarized gas target for LHC
Conclusions

2. Motivation for internal targets
Study of spin dependent observables:
Polarized beam - extracted from accelerator, or secondary beams
on a polarized target – by dynamical nucl. polarization = microwaves in high B at low T
(SLAC experiments E…, European Muon Collaboration EMC/CERN, Spin Muon Collaboration SMC, COMPASS….)
New idea (1980‘s): Stored beam - multi-mA plus polarized gas target: HOW??
Atomic polarized H and D beams as target:
Studied at Stanford Tandem with mA beams
Experiment at VEPP-3 (Novosibirsk)
t ~ 2·1011 /cm2 
Polarized atomic beams: produced by an ABS
CERN
Polarized gas target for LHC

3. Atomic Beam Source (ABS)
Collimated cold atomic beams produced by dissociator with cold nozzle (100K) and differential pumping system (Stotal ~104 l/s)
Spin-dependent focussing of H and D atomic beams by 6-pole magnets: mJ= +1/2
Nuclear polarization Pz, Pzz produced by means of rf transitions with ~100% effi-cieny + rapid switching
Bc = 50.7 mT
CERN
Polarized gas target for LHC

4. Free Atomic Beam as a Target?
Best example: the polarized H-Jet at RHIC used as absolute Polarimeter for the proton beam(s):
A beam from an ABS is focused on a bright spot shortly after the last Sextu- pole magnet and employed as target, surrounded by detectors. The ABS intensity is 12.4·1016 H↑ /s (about 2× intensity of HERMES-ABS) and the areal density seen by the proton beam is t  /cm2 .
With the LHC proton beam – Ip = /s – the luminosity would only be /cm2 s*.
This is the maximum available density for a free polarized ‚jet‘ with present technology – despite all the effort of many decades!
*) note: the gas load of a jet for a given target thickness would be high compared with a storage cell target.
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Polarized gas target for LHC

5. Compression Tube (CT) for intensity measurements
AAbggbkA
ABS
Gas conductance C (molecular flow) of tube – length L and i.d. D:
C = 3.81 l/s √(T/M) × D3/(L+4/3 D)
Example: T=100K, M=1, L=10cm, D=1cm
→ C = 3.36 liter/sec
I (part./s into given phase space)
r = I / C CT
e.g. r = 6.5∙1016 / 3.36∙103 cm3 = 1.93∙1013/cm3
The pressure p0 at the tube end is related to the intensity I by
I = r ∙ C = p0 C / kT
From the measured p0 and T, and the calculated Ctube, the intensity I (part./s) can be deduced.
CERN
Polarized gas target for LHC

6. History of Storage Cells
Storage Cell to enhance the target density?
Storage Bulb of Hydrogen MASER (Ramsey 1960) with Teflon coating
Storage Cell for ion beams (proposed by Haeberli 1965, demonstrated 1980 by Madison group with Tandem beam)
FILTEX proposal to use Storage Cell targets as Spin Filter for stored antiproton beam (Heidelberg group 1985). The cell is a thin-walled T-shaped tube with straight beam tube and feed tube from the side. Polarized gas is injected by an ABS ballistically via feed tube into the cell center and diffuses outwards through the three openings, performing in average about 300 wall collisions.
Estimates based on molecular flow: Factor ≥ 100 expected! 
Storage cell has one big advantage - density - and many disadvantages – wall depolarization and recombination, extended target, material close to beam, …. But no alternative!
CERN
Polarized gas target for LHC

7. Storage Cell for density enhancement
AAbggbkA
Central density r0 with T-shaped cell (C2 = conductance of feed tube):
Conductance C1 of beam tube
ABS
L1 = 25cm, D2 = 1cm, T = 100K, M = 1 (H):
C1 = 1.45 l/s
r0 = I/ (C2 + 2 C1) = / cm3
= /cm3 I
I = 6.5∙1016 H/s
HERMES
feed tube D1
beam tube
ion beam L1 L1
Target (areal) density
t = ½ Lbt ∙ r0 = L1 r0
= /cm2
at 100K and Lbt = 50cm
Volume density r0
-L1 L1 z
Note: this areal density t is for a 50cm long beam tube with i.d. of 1cm, at T=100K and fed by a HERMES-like ABS. Other dimensions and assumptions result in different numbers. Guidance needed for a reasonable lay-out.
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Polarized gas target for LHC

8. 1992: Target test in the Heidelberg Test Storage Ring TSR
experiments
Ring designed for Heavy Ions – no shielding. Four straight sections: Injection (foreground), Electron Cooling (left), Experiments (background), Cavity and Diagnostics (right)
Polarized gas target for LHC
CERN

9. FILTEX target Target installed in the TSR (1992) beam 2017-01-10-CERN
Polarized gas target for LHC

10. FILTEX target: view into target chamber
Picture taken through Kapton detector window (chamber under vacuum)
Last 6-pole magnet focusing H atoms into storage cell
T-shaped storage cell with straight beam tube 10 mm i.d.
Cooled cell support and Cryo pump
CERN
Polarized gas target for LHC

11. FILTEX target First results (1992)
Asymmetry with stored a beam
Target polarization P = 0.8 (single substate) at 100 K
The FILTEX target was employed for the HERMES experiment at the electron ring of the HERA ep Collider (DESY)
CERN
Polarized gas target for LHC

12. HERMES H&D Target- Overview
ABS: Dissociator with cooled nozzle and differential pumping; permanent 6-poles and RF transitions
Target chamber with cell, holding field coils, beam and sample tubes
TGA (Target Gas Analyzer): Measurement of dissociation degree a
BRP (Polarimeter for atoms): Measurement of substate population of atoms → electron pol. Pe and nuclear pol. Pn !
Sampling corrections applied to calculate polarization seen by the beam
Target chamber
CERN
Polarized gas target for LHC

13. HERMES Detector with H&D Target (1996-2005)
1996/97: successful run with ‚longitudinal‘ hydrogen! Running and analysis procedures had to be developed.
1998/2000 longitudinal deuterium running (Pz and Pzz)
2002/05 transverse hydrogen; important for possible measurement of Single-Spin asymmetries at the LHC. Machine conditions in Run 2 difficult (more spin rotators, comp.-solenoids removed): P┴ = 0.80±0.03
Side view. blue: spectrometer magnet with back chambers, Cerenkov, TRD and lead glass wall. The H&D target and front detectors are shown on the right.
CERN
Polarized gas target for LHC

14. Polarized gas target for LHC
Storage Cell Design
Liverpool-Madison-Ferrara
Cell optimized for operation in an electron storage ring (wake fields, SR in the kW range)
Conducting surface with smooth variation of cross section to avoid excitation of wake fields!
System of W collimators for protection against beam and SR
Cooled via cooling rails with cold He gas to K
75mm Al walls with Drifilm coating - Radiation damage visible!
But: Very effective wall coating due to ice layer maintained by small fraction of water in the atomic beam !
CERN
Polarized gas target for LHC

15. Diagnostics: Target Gas Analyzer
Target gas analyzer (TGA)
- measures degree of dissociation a to 1% in few minutes
Temperature scan on used cell (deuterium 2000) after formation of water layer
no T-dependence visible:
no recombination in the (perfect) cell!
CERN
Polarized gas target for LHC

16. Diagnostics: Sampling Polarimeter BRP
* measures substate population ni of sample beam to Dn/n = 1% in few minutes n
Hydrogen hfs-population as function of holding field
Detector
Sample Beam
From the ni and a, and by applying ‘sampling corrections’, the target polarization
as seen by the beam is calculated. The relative error achieved is about 4%.
CERN
Polarized gas target for LHC

17. Polarized gas target for LHC
PAX target at COSY
Former HERMES target re- configured and improved (A. Nass et al., FZJ, and G. Ciullo et al, INFN Ferrara). Employed for the PAX experiment: Spin Filtering of stored protons in COSY by a polarized H target (2011/12), in preparation for Spin Filtering of anti-protons at FAIR.
Shown: Target installed in the COSY tunnel at the PAX place in between four low-b quads (blue). ABS (top), target chamber with openable storage cell (center) and support for the target polarimeter (left).
CERN
Polarized gas target for LHC

18. Openable storage cell development in Ferrara
Storage Cell for 2 GeV p/d beam at COSY (FZ-Juelich)
Length: 400 mm
Free diameter in open/closed position: 10 / 40 mm
closed
open
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Polarized gas target for LHC

19. Polarized Gas Target for LHCb
nnn
A storage cell gas target is the most efficient method in terms of (areal) density t vs. gas load.
The density t depends on input rate I of the ABS (or unpol. gas feed system), and the geometry and temperature T of the cell, and can be varied in a wide range.
The HERMES electron scattering experiment was limited by the available target density. Therefore, a huge effort went into improving the ABS intensity. The range of densities for a polarized gas target for LHCb have to be specified in order to come up with reliable performance figures.
As a guess, we may assume the following:
* A storage cell with L = 50cm (i.e. L1 = 25cm) and i.d. of D1 = 1.4cm at T = 100K, and the standard feed tube 10cm long and 1cm i.d.
With the HERMES ABS intensity of I = /s, we get t = 1.44∙1014/cm2 ≈ t HERMES
With Ip LHC = 3.63∙ TeV, the maximum luminosity is Lmax = 5∙1032/cm2 s
Relative loss rate of the 7TeV beam: < 10-7/s, i.e. no effect of the target on the proton beam life time!
Conclusion: a HERMES/PAX type gas target with conservative assumptions on cell geometry (D1 > DSMOG) enables to produce target densities of the order 1014/cm2. The value can be adjusted according to the data taking strategy of LHCb.
CERN
Polarized gas target for LHC

20. Further considerations
A gas flow of about 1017 atoms/s corresponding to Q∙(H2) = 1.9∙10-3 mbar l/ s into the target section must be accommodated which requires a powerful differential pumping system.
In case of a failure of the target source and/or the polarimeter, the valves to the target section will be closed by the Interlock. In this case, the option to feed the cell with unpolarized gas for a different physics program is possible. Access to the target area for fixing the polarized target should be enabled in due time.
The SMOG system has paved the way towards Fixed Target data at the LHC. The proposed storage cell target with differential pumping could add new options also with unpolarized (heavy) target gas. Different combinations of masses could be studied, e.g. Pb on Xe or on Kr. For the Pb beam, the nucleon-nucleon CM energy is √s = 71.9 GeV, above the values of the CERN SPS Heavy Ion program (about 20 GeV) and the FAIR CBM program (4 – 9 GeV).
CERN
Polarized gas target for LHC

21. Polarized gas target for LHC
Conclusions
A Storage cell target gives highest areal density at minimum gas input. Good performance over many months experienced at the HERA 27.6 GeV electron storage ring in ten years of operation (1996 – 2005) at e± currents up to 40 mA. A cell of 50cm in length and 1.4cm i.d. has been assumed, in accordance with the aperture requirements at the SMOG/VELO detector of LHCb.
Polarized H and D gas target by ballistic injection of polarized gas from an ABS: (i) luminosities of the order 1032/cm2 s seem accessible; (ii) practically no back- ground i.e. better systematics compared with a solid polarized target; (iii) sign of P switchable at 1/min or faster. The density can be adjusted to eventually allow for data taking in parallel to the pp collider programme.
Cell filled with unpolarized gas: p-A and Pb-A collisions could be studied with gases like H2, He, Ne, Ar, Kr and Xe at √sNN = 72 GeV at the optimum luminosity.
Next: (i) simulations to study and optimize the layout, and (ii) a conceptual design. This may finally lead to measurements of Single-Spin Azimuthal Asymmetries of various processes at the LHC.
Thank you !
CERN
Polarized gas target for LHC