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EXPERIMENTS WITH DENSE POLARIZED INTERNAL TARGETS
E. Steffens
To cite this version:
E. Steffens. EXPERIMENTS WITH DENSE POLARIZED INTERNAL TARGETS. Journal de
Physique Colloques, 1990, 51 (C6), pp.C6-221-C6-230. �10.1051/jphyscol:1990618�. �jpa-00230884�
COLLOQUE DE PHYSIQUE
Colloque C6, suppl6ment au n022, Tome 51, 15 novembre 1990 EXPERIMENTS WITH DENSE POLARIZED INTERNAL TARGETS
E. STEFFENS
Max-Planck-Institut fiir Kernphysik, Postfach 103980, 0-6900 Heidelberg, F.R.G.
Resume - Deux expbriences basees sur des targets denses polarises de nucleon pour des anneaux de stockage sont discutks. Dans ce papier le status present du FILTEX target et les considkrations du 'design' pour le HERMES target sont presentis.
Abstract
-
Two experiments are discussed which are based on dense polarized nucleon targets for storage rings.In the paper the present status of the FILTEX target and design considerations for the HERMES target are presented.
1- INTRODUCTION
Storage rings, or storage ring accelerators, are of increasing importance in hadronic physics. This is partly because of the high c.m. energy achievable in the collider mode, and partly because of other advantagous options of storage rings like phase space cooling, pulse stretching and accumulation of rare particles.
Circulating currents in such rings are of the order of 1016 to 1018 part.1~. Therefore, with an internal target of 101~1cm20r more in density, one may already obtain useful luminosities of to 1032/~m2 S. A review of internal targets for storage rings has recently been given by Ekstrom/l/.
The production of polarized atomic beams of hydrogen and deuterium is a well established technique from polarized ion sources. Densities up to 1012/cm2 have been reported 121, which is already sufficient for some internal target applications. The density may be increased by injecting the atoms into a T-shaped storage cell with thin wall 131. Depending on the geometry, density enhancement by a factor up to 100 is expected (fig.1). Disadvantages are the presence of walls close to the beam axis, the large number of wall collisions and close encounters with the ion beam, which may depolarize the target gas, and the extended target size. Advantages of these targets are their purity, rapid reversal of polarization and flexibility in the choice of polarization components.
Why are experiments with dense internal targets in storage rings interesting? This is motivated by some surprising new results from different fields:
i) The measurements of Ann in pp inclusive scattering at large pt 141. For extension of these mea- surements at a large proton storage ring (UNK-Protvino) a new internal target is being developed, based on an ultracold hydrogen source 151.
ii) The measurement of the spin-dependent strucure function g1 (X) of the proton in deep inelastic polarized muon scattering by the EMC collaboration 161. The result is in contradiction to the Ellis-Jaffe sum rule and it has deeply affected our understanding of the proton spin structure. A test of the fundamental Bjorken sum rule requires a careful measurement of the neutron structure function and a re- measurement of the proton result with improved accuracy.
This is the aim of the recently proposed HERMES experiment at DESY /7/. Polarized electrons of up to 35 GeV will be scattered from an internal target (G,
a
and 3fie for additional neutron measurements) and detected in a spectrometer. At present the faisibility of this experiment is investigated.iii) The anomaly of the p-parameter in pgscattering at low momenta /g/. For further studies the mea- surement of spin-dependent cross sections at low energies has been proposed (FILTEX 190, which ideally requires polarized target and beam. The proposal is based on an internal polarized hydrogen target, operated in the LEAR ring at CERN.
In the next section, the target requirements are discussed with HERMES and FILTEX as examples. Then the development for the FILTEX target source is described and the expected performance estimated. The last section is devoted to the discussion of how such targets are integrated into a storage ring, which problems arise and how they may be solved.
2 - TARGET REQUIREMENTS
A general list of requirements for a polarized storage ring tyget has certainly to include the following topics:
- High density n (atoms/cm2 ) and polarization PT.
-
Precise determination of poiarization by a polarimeter which measures continously the target gas.- Sufficient aperture for the beam in order to reduce the background from beam halo.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990618
C6-222 COLLOQUE DE PHYSIQUE
Fig. l Schematic representation of a polarized internal target experiment with storage cell.
POLARIZED HYDROGEN SOURCE
Fig. 2 FILTEX target with target chamber and atomic beam source.
- Thin, large area windows (and thin walls of storage cell) for the passage of scattered particle with low straggling.
- Differential pumping system to protect the ring vacuum.
- Minimum disturbance of stored beam, e.g. minimum coupling indepances for bunched beams as in electron machines etc.
This list could easily be extended, but instead we will illustrate the target requirements by discussing the two examples already mentioned. The FILTEX target represents dense targets in a low-energy ion cooler ring, whereas the HERMES target is dedicated for a high-intensitv, high-energy electron storage ring with narrow bunches.
FILTEX: The aim of this experiment 19, 101 is to polarize antiprotons circulating in LEAR via spin-dependent attenuation by the target. Spin-dependent reactions between beam and target are studied using a detector system which surrounds the target.
The total strong interaction pp cross section may be written as:
--+ ---f
u t = CO
+
u ~ P B - P ~4 (1)
Here PB is the beam polarization and 00 (cl) the spin- independent (-dependent) fraction of ot. If we neglect depolarization of antiprotons in the storage ring, the build-up of beam polarization is given by:
t Cl t
P ( t ) = tanh - = tanh --
7 1 C O T O
and the intensity: I ( t ) E e-tlTo (for a 1 < < ao) (3)
The time constants are: Polarization build-up-time: r1 =
-
1n
f
0 1 (4a)Beam life-time 1
TO =
-
n
f
CO (4b)with f = revolution frequency. For a beam life time zo = 10 h (only strong interaction considered) a target density of n = lO"+/cm2 is required
.
In practice the beam life time will be reduced via spin-independent losses, e.g. neu- tralization or scattering from the target or residual gas.The key question is how one can establish conditions for long storage times in the order of several hours and low depolarization. Necessary requirements are 11 1, 121:
-
Electron cooling for compensating multiple scattering, - Large ring acceptance at target position,- Compensation of longitudinal B-field in the cooler.
A test experiments with protons is in preparation at the Heidelberg Test Storage Ring (TSR) 1131, which will probe the accelerator physics questions involved. It should be noted that this experiment totally depends on the availability of an internal target and a strong cooling system.
A schematic cross section of the FILTEX target 1141 is shown in fig. 2. The atomic beam from the source is injected into the T-shaped stora e cell in the center of the spherical target chamber. B the tilt angle of 30' of the source interference is avoide% between the atomic beam diagnostics and the &ur hodoscopes arranged symmetrically in the horizontal and vertical plane.
A polarized atomic beam is produced by means of two sets of sextupole magnets and an rf-transition (state 2 into 4) in between in order to separate the pure state Im, m, > = I 112 112 >. For the experiment constant vertical polarization is required. A weak vertical guide field is provided by a pair of coils. Reversal of polarization is obtained by changing the sign of the guide field.
The thin aluminium walls of the target cell are coated with teflon in order to minimize depolarization 115, 161 during the numerous wall collisions (
-
400). The polarization of atoms emerging from the upstream end of the beam tube will be analyzed continously by a Balmer-polarimeter 114, 161, which is based on the analysis of circular polarization of Balmer photons from hydrogen atoms excited by slow electrons.The target for the TSR test experiment and the target source will be described in more detail in sec. 4 and 3, resp.
HERMES: This experiment was recently proposed for the HERA electron ring 171. Electrons are polarized by the Sokolov-Temov effect and rotated into the longitudinal direction by a pair of spin rotators. They are scattered from a polarized internal hydrogen, deuterium or 3He target and detected in a magnetic spectrometer in the angular range from 40 to 250rnrad. Due to this extended kinematical range the structure functions can be measured in the range of Bjorken variable X from 0.002 to 0.8. Therefore, not only the integral of g,(x) over X, but also the function itself can be measured precisely.
Comparing the HERMES figure of merrit with that of the old EMC experiment, it appears that the same quality of data is obtained after 10 h of running only, in contrast to 100 days of EMC running time. In addition to g, (X), new structure functions g,(x) for the proton and neutron can be measured with transverse target polarization. For the deuteron target, also the spin structure functions b (X) and A (X) are accessible. As pointed out by R. Jaffe in his talk, these measurements are required to complement our picture of the nucleon structure and to provide information on the spin structure of the lightest possible nuclear target, the deuteron.
Two other experiments have been proposed to study similar physics, but with different methods.
i) SMC (CERN) 1171: This experiment uses the same muon beam and detector as EMC 161, but an improved solid deuterium and proton target. Remeasurement of the proton and a new measurement of the deuteron is foreseen, from which the neutron spin structure can be deduced.
ii) E-142 (SLAC) 1181: It is based on the SLAC polarized electron beam and a high-pressure polarized 3He target. Because polarized 3He is represented in a certain approximation by two saturated proton spins and one neutron spin, the neutron spin structure function can be extracted after some nuclear corrections.
A comparison of the three proposals shows that E-142 may provide the first measurement of the neutron. SMC will start with measuring the deuteron, using the old EMC target. Later a new target set-up with improved field capacities and new NMR system will be used. HERMES is the most precise and complete experiment , but depends on the availability of electron polarization at HERA, which is still an open question 1191. SMC is already approved and preparing for the experiment, whereas HERMES will not be approved before 1991 because of the required tests of electron polarization.
Both for FILTEX and HERMES, high target polarization of 0.8 or more and high density of about 1014 atoms/cm2 is required. The density of a storage cell target is determined by a flux of atoms from the source, and the geometry and consequently the gas conductance of the cell in the molecular flow regime 131. In both cases the cell geometry is such that the source has to deliver about 1017 atomsls in flux. Because of the low guide field in the FILTEX target, this figure holds for a single substate of hydrogen. This is higher by about a factor of 5 compared to the flux from present atomic beam sources.
Therefore a program to investigate and overcome limitations of atomic beam sources was performed by the FILTEX collaboration. The present status is presented in the next section.
3 - FILTEX TARGET SOURCE /14,20,21/
The source (fig.2) is optimized for the highest possible flux into the acceptance tube (1 cm diam, 10 cm long) of the target cell. This differs from the design goal of cold sources 121, which aim at high volume density, but at the expense of a larger divergence.
C6-224 COLLOQUE DE PHYSIQUE
In order to meet the design goal of 1017 H/s in one substate, dissociator and differential pumping system were
-
designed for high gas flow up to 5 mb Vs of H,. Extensive measurements and optimization on beam formation and atomic beam parameters have been done. Based on these results, a system of sextupole magnets was designed 122,201 and ordered in industry. The magnets are of the type of segmented permanent magnets 1231 and of very compact design. After delivery, final tests of the complete source and intensity measurements will be performed.
The differential pumping system includes two skimmers and turbomolecular pumps for the first two stages, with 6000 and 2000 Vs H2 pumping speed, resp. It is followed bv two chambers with turbomolecular and cryogenic pumps for the magnets and the rf-transition. The last magnet is located inside the target chamber in front of the feed tube.
The dissociator is similar to the SIN type 1241, but with a larger pyrex tube and a cooled aluminum nozzle 1251.
Atomic beam measurements were done using a quadrupole mass spectrometer with 10 vs rise time. Intensity,and degree of dissociation were determined by applying a slow chopper and lock-in amplifier. Intensities of H2 beams were also determined using compression tube and ionization gauge.
Attenuation by residual gas: Hydrogen gas (molecular or partly dissociated) flows through the cooled nozzle (temperature TN) and a beam is formed by expansion and collimation. After expansion the beam may be described approximately by a cosn e -distribution.
Here Q is the total flow. The forward intensity (e =0) is increased by a factor (n +1)/2, compared to a cosine distribution. Therefore n can be measured by comparing absolute values of flow Q and intensity d I on axis. It is also related to the Mach number M of the beam. Compared with typical nozzle beam conditions 1261, the pressure at the nozzle is low
(c:
0.5 mb). Nevertheless expansion effects are clearly visible like narrowing of the velocity distribution and peaking of the flow distribution /20, 271.With increasing flow Q, the gain factor (n
+
1)/2 should increase. Therefore the forward intensity is expected to increase more than linearely with Q. On the contrary if the intensity increases less than linearly with Q, this is a clear manifestation of residual gas scattering. We have used this method to identify such losses and to minimize them.An example for TN= 300 K is shown in fig. 3. Closed circles represent the measurements without any obstacle near the beam axis, which may hinder the flow, e.g. with empty vacuum chambers. It deviates from a linear in- crease for a gas throughput above 3 mbVs. For nozzle temperatures lower then 300 K this effect is even much stronger. The attenuation takes place in the different stages of the differential pumping system, i.e. the dissociater chamber (l), the skimmer chamber (2) and the magnet chambers (3
+
4, the numbers refer to fig. 2). It can be de- scribed quantitatively by a model with total flow rate Q, gauge pressures pi (i= 1-4) and attenuation cross sections as input. Best agreement is obtained for a gain factor of 3, i.e. a cos5edistribution (n=5).In addition, we have studied the attenuation by the pressure bump which is produced if the flow close to the beam axis is restricted, e.g. by the bore of sextupole magnets. In the case of segmented permanent magnets /22/ selected for the FILTEX source, the closed bore is of cylindrical shape. For the study of beam attenuation, magnet dummies were used consisting of tubes and discs. Due to the d3-dependence of the conductance of tubes on the diameter d, strong attenuation is expected for narrow tubes. Experimentally it has been observed, that for an H2 beam (QH~ = 5 mbVs) and a tube of d = 2 cm and 1 = 20 cm the intensity with dummy was reduced to 113 (116) of the free beam intensity for the nozzle at TN = 300 K(50 K). This drastic effect shows that flow restriction introduced by magnets and other elements close to the nozzle may determine the maximum intensity much more
0 1 2 3 4 5
H2-THROUGHPUT [mbar I/s]
0 I 2 3 4 S
Gas Flow (mblls)
Fig. 3 Forward intensity of a molecular hydrogen Fig. 4 Degree of dissociation of the atomic beam as function of flow without (full circles) and with beam as function of gas flow.
(open circles) magnet dummy.
than e.g. the pole tip field. That is why the diameter of the magnet bore was chosen to 2.4 cm and the first magnet was split into a short (5 cm) and a long section (15 cm) , separated by a 2 cm-gap to improve the pumping conditions. Intensity measurements with magnet dummy of this size are shown in fig. 3 as open circles. At Q = 3 mbVs, the additional attenuation caused by the dummy is only 20 % (H2, 300 K).
Degree of dissociation a : Measured values of this quantity depend very much on definition and detection method. It is used to describe the fraction of atoms in the beam. The fraction of HI or H2 may be described by the volume density p1 and p% as measured by a quadrupole mass spectrometer (QMS), or by the intensity i l and i2, which is density times velocity.
The measurement which we consider as most reliable were done according to the definition:
Here P1 and P2 are determined by the QMS with crossed-beam source. Of course we have to know the relative calibration factor for H1 and H2.
In fig. 4, a is shown as function of gas flow Q( Hz) for TN= 150 K. The upper curve was measured using a Faraday cup as ion detector, the lower curve with multiplier, for which the detection efficiency may depend on the ion velocity, i.e. the ion mass. Therefore we consider the upper curve of fig. 4 as the most reliable measurement of a. The relative difference between the two curves is about 20%.
The measured a reaches nearly 70% at 1 mbVs, i.e. for a flow value which is already a factor of two higher than for cold sources 121. It falls off to about 40% at 3 rnbus, where the maximum density for atomic hydrogen is obtained. In the range of nozzle temperatures between 330 and 150 K, a increases linearly with decreasing temperature.
Therefore a further improvement is expected below TN = 150 K. We also observe a strong and reproducible effect of a small admixture of water or oxygen to the hydrogen gas, which increases u by a factor of 1.7 to 1.8. This effect has been explained as a catalytic dissociation of OH molecules on the pyrex surface /28,20/. An optimum admixture of water has been used in the measurements of fig. 4.
Velocity distribution: For the design of the magnet system the velocity dismbution of the atomic beam is required. Because of the high gas flow of the FILTEX source, it seemed inappropriate to assume the same beam parameters measured for much lower flow values /27,2/.
Fig. 5 Time-of-flight specttutn of a molecular hydrogen beam (see text).
The measurements 1291 were performed by the time-of-flight method. A fast chopper, rotating at 280 Hz, was used to produce 50 ps long pulses. Their time spectrum was measured by the fast QMS after a 55 cm flight path.
The huge background from residual gas was substracted after signal averaging. A typical time-of-flight spectrum of the atomic hydrogen beam is shown in fig. 6. The curve H(t) is a fit to the measured data points. F(t) is the TOF spectrum, unfolded with the chopper opening function. From this curve, the parameters VD (drift velocity) and TS (beam temperature) are extracted, which determine the velocity distribution f (v):
f ( v ) = cl
.
v 2.
e z p[
-- 2LT. m (v -A variety of velocity measurements for molecular and dissociated beams as function of gas flow and nozzle temperature has been performed. The results are as follows 1201:
1. Pronounced nozzle effects are observed at lower nozzle temperature. Compared to the Bonn measurements 1271, which where done at a six times lower gas flow, the drift velocity is considerably higher (+25%) and the beam temperature lower by a factor of about three.
C6-226 COLLOQUE DE PHYSIQUE
2. Due to the admixture of molecular hydrogen in a dissociated beam, mixing effects occur which tend to slow down the atomic component and accelerate the molecules, thus modifying the velocity distribution.
3. At the working point of TN = 140 K and QH2 = 3 mbVs the following HI beam parameters were found:
4. Measurements of the beam profile of the collimated beam at large distance indicate that during expansion a highly-parallel flow is produced, thus confirming the picture of a supersonic beam.
Expected performance: Based on these measurements, the beam intensity I accepted by the target cell can be
estimated: 1 AR
I = - . Q H ' - . T . A
4 R (8)
The factor 114 takes into account that one substate only is employed. QH is the effective flow rate of atomic hydrogen (mm. 6 X 1019/s). r n ~ is the relative accepance solid angle as inferred from trajectory calculations for the optimum magnet system ( A ~ Q = 0.02 for the cold beam), T is the transmission through the optimized system of four sextupole magnets, as inferred from a Monte Carlo simulation of trajectories. T depends strongly on the initial phase space distribution of the beam. For the two limiting cases of "effusive" and "highly-directional" beam T varies between 0.2 and 0.6. A is a factor taking care of attenuation in the magnet bare due to scattering (A
-
0.6). The expected intensity is: I = (7 f 4)
.
1 0 1 6 / ~The uncertainty is mainly due to the assumptions which enter the transmission T.
4 - PROTON TEST EXPERIMENT
The TSR is a low-energy , large-acceptance storage ring with large diameter UHV chambers and a base pressure of 10-l1 mb 1131. It is equipped with an electron cooler. Intense stored beams are obtained by combining different stacking techniques 113,301. A low-p mode has been developed, using the normal set of 20 machine quadrupoles but grouped in seven instead of five families. Beta values of 0.3 to 0.35 m are obtained and the angular acceptance at the target position is 17 to 15 mrad.
From these figures, some basic target parameters follow. As target cell a thin-walled A1 tube with inner diameter 11 mm and 250 mm in length will be used. It should be noted that the full acceptance of the machine in the low-p mode is not affected by this tube.
The losses due to single scattering from the target are described by the cross section:
r
Here oc (8) is the lab Coulomb cross section and 8, the acceptance angle. For 8, = 15 mrad and 30 MeV protons we have &ac = 0.3 b, i.e. the same order of magnitude as the cross section for strong interaction.
The life time of a stored proton beam of 21 MeV has been measured with continuous electron cooling. For the empty machine in normal mode a life time of 36h was found 1301. This life time is reduced by about a factor of four in the low-p mode due to the reduced acceptance. Tests have also been done using a tube of the same dimensions as the target cell (11 mm i.d., 250 mm long). More than 1mA of proton beam was accumulated into the narrow tube with no significant change of beam life time 1311.
For the success of FILTEX, long polarization life time zp in the ring in the order of several hours is required. For finite 2, the polarization build-up as given by equ. 2 is modified 191. It increases more slowly and saturates at a finite value PB 1. For z, >> ~p (see equ. 4a) it can be derived: PB(t -+ oo) = . r , / 2 ~ i
Due to the high spin-dependent part of the total pp cross section the polarization build-up time for the TSR experiment will be 1 G 25 h. What do we know about 7, ?
Due to a series of beauftiful experiments with polarized protons in the IUCF cooler 1321, our knowledge about storage of polarized ion beams is growing rapidly. Recently, protons were stored for 10 minutes and their polarization measured and compared with freshly injected protons. No significant loss of polarization was observed 1331, telling us that for a well tuned ring the relation holds: 7, D 10 min. We hope to establish similar favourable conditions in the TSR by compensating the longitudinal B-field from the cooler to 1-2% and by a careful choice of the working point, in particular the vertical tune Qz, in order to avoid spin resonances 132,341.
Target installation: A cross section of the target chamber is shown in fig. 6. The target cell is of clam shell design. The A1 wall is 0.5 mm thick and coated with Teflon. It can be cooled to temperatures below 100 K using a closed-cycle refirigerator in order to enhance target thickness. The polarization of the target gas is monitored by the
"Balmer-polarimeter" located close to the upstream opening of the target cell 1161. A weak guide field in the vertical direction is provided as target spin axis. By reversing it, the sign of the target polarization is changed.
FILTEX Target Chamber
Detector box o
Fig. 6 Target chamber with storage cell, detectors and vacuum system.
The chamber is pumped by a large built-in cryogenic pump with about 104 Vs pumping speed. The atomic beam corresponds to a gas flow of about 10-3 mb Vs, which will result in an H2 pressure of 1-2 , 10-'mb. On both sides of the target chamber three differential pumping stages are provided wi@ movable collimators to reduce the gas flow. In total 32 NEG modules are used together with two 200 Vs ion getter pumps. We estimate that at the end of the straight section, about 3 m from the target center, a pressure of 10-10 mb is obtained which is compatible with the standard TSR vacuum system 1131.
The detection of low-energy protons from a UHV scattering presents a difficult problem. In addition we are dealing with an extended, 25 cm long target, i.e. a large-area detector at a distance from the target is required in order to obtain a certain angular resolution. Therefore the target chamber 1351 has two symmetrical 250 mm flanges in the horizontal and two 200 mm flanges in the vertical plane, as close to the forward direction as possible (q,, 2 200). Four plastic scintillator hodoscopes are located in evacuated detector boxes, separated from the UHV by large area Kapton windows. They consist of 5 cm wide AE stripes and a thick stop counter. The performance of the detector system and the influence of kinematical broadening and straggling in the wall of the storage has been studied by means of Monte Carlo sirnulations. The results show, that with 1mA circulating current and 1014
+
Hlcm* target thickness high count rates of several kHz per detector occur. About 30% of the events are kinematical coincidences, i.e. events, which are nearly free of background from scattering in the cell walls. With these high rates even small count rate asymmetries can be measured in a short time.
The whole TSR target installation is presently being set up and tested. Installation into the ring is scheduled for fall of this year.
5
-
HERMES TARGETAs discussed before, the HERMES target/7,21/ will consist of a storage cell, supplied by different target sources.
Here the FILTEX ABS (see sec 3) may be used for hydrogen or deuterium. As possible alternative the spin-ex- change optical pumping source is studied at ANL 1361. For 3He the CALTECHIMIT source 1371 has been proposed, based on optical pumping of metastable 3He 1381.
The target installation will be part of the HERA electron storage ring and has to fulfil1 many requirements. New aspects, compared to the FILTEX target, are the occurence of intense synchrotron radiation and rf-fields from the short electron bunches. In addition, there is no easy way to monitore the target polarization by means of scattering events. Therefore a sampling polarimeter based on a Stern-Gerlach magnet and rf-transitions is being studied, capable of measuring the target polarization to about 3% 139,401. Some new aspects of the HERMES target are discussed in the following.
HERA Electron Ring: It is part of the electron-proton collider HERA at DESYIHamburg. The machine will provide 30 to 35 GeV electrons in 210 bunches at a bunch frequency of 10.41 MHz and an average current of 60 rnA. The bunch length is about 30 ps, resulting in a peak current of more than 200 A.
The machine has four 360 m long straight sections. Two sections are taken by the collider experiments ZEUS and HI. The West hall is filled with injection and other machine components. The East hall will be used to test the first spin rotator in the same configuration as used for the collider experiments. For the HERMES experiment it is proposed /7/ to straighten out the electron beam line, after the rotator tests have been done. In this way the electron and proton beam will be separated by nearly 1 m which is enought to avoid interference between the proton beam and the electron spectrometer.
The electron polarization 1191 is produced by means of the Sokolov-Ternov effect. As the polarization build-up time is proportional toy -5, high energies are of great importance. Therefore polarization tests are only meaningful with superconducting cavities in order to boost the energy to 30-35 GeV. The f i s t tests are planned for early
C6-228 COLLOQUE DE PHYSIQUE
A B S
1991. A compton polarimeter is under construction.
Target Polarization: The system required for the production and detection of deuterium polarization is shown in fig. 7. A deuterium atomic beam in a single substate produced by means of 6-pole magnets and a set of two- level transitions. Before injection into the target cell, the sign of the vector polarization is reversed by a weak field transition. The target gas is stored in the cell, which is traversed by the electron beam. A strong guide field is applied in order to conserve target polarization 1401. A sample atomic beam extracted from the target cell is analyzed by the rf-spectrometer, consisting of a variable rf-transition, 6-pole magnet and sensitive beam detection system. Such a system is presently under construction at Heidelberg. It will be applied to study wall depolarization in a strong guide field for the fist time 139,411.
I
h I
Target 6 '2
Chamber
beam
Monte Carlo simulations are performed for the molecular flow of the target gas. As results average numbers of wall bounces and the spatial distribution of polarization are obtained. Also the ratio of sample beam to target polarization is calculated. The results show that for a high target polarization spin-flip probabilities per wall bounce of less than 10-3 required 141,421.
RF Fields: Due to the short electron bunches of 30 ns in length, a broad frequency spectrum with harmonics of the bunch frequency f, = 10.41 MHz is generated by the beam. This may result in:
-
Exitation of resonant modes of any conducting structure traversed by the beam ("rf-heating").-
Resonant depolarization of the target atoms 140.421.The effect of rf-heating has been studied for different target geomemes /43,7/. Heating of the target cell is due to non-resonant wall currents and will be less than 4W. Resonant heating of the target chamber is within acceptable limits.
Buqch field depolarization has been calculated in different approaches 144, 45, 71. In the most advanced calculation the diffusion of the target atoms in the cell is taken into account 1451. At a guide field of 0.325 T, which is in the middle of two "resonant" field values, the average depolarization for hydrogen was calculated to 12% and for deuterium to 2%.
Fig. 7 Arrangement required to polarize and analyse deuterium
target gas.
Synchrotron Radiati0n:Photons with energies up to several hundred keV from the upstream dipoles and quadrupoles are hitting the target section. If scattered into the detector, they would result in a tremendous background rate. Therefore collamators will be used in order to cut this intensity down to an acceptable limit.
Calculations have been performed /46/ in order to design a suitable system of collimators. As a result, a system has been proposed / 71 which consists of two pairs of collimators, one acting as main collimator, and the second designed as anti-scattering collimator. The dimensions of the target cell are chosen such that it is shielded by the collimators with respect to primary and scattered X-rays.
6
-
CONCLUSIONSI have tried to show that internal polarized nucleon targets with 10~4atoms/cm* would enable completely new experiments which are otherwise not feasable. For hydrogen and deuterium the combination of a conventional atomic beam source, pushed to the limits of this concept, and a storage cell target may produce such a density.
Other approaches presently under study are a free ultra-cold hydrogen beam as target, or a spin exchange optical pumping source coupled to a storage cell.
The FILTEX target, which consists of a high intensity atomic beam source and a storage cell target, is nearly ready for installation into the Heidelberg test storage ring. The operation of the first high-density target will provide valuable information on limits and chances of such experiments. In addition, we are able to investigate a new method to polarize particles circulating in a storage ring, which eventually may lead to experiments with polarized antiprotons and polarized internal target in LEAR.
For the HERMES experiment, the same technique is proposed, but adopted to the specific requirements of a high energy electron ring. Our simulations and the recent experience from the Novosibirsk-ANL internal target experiment tells us that the most obvious problems are under control. In a few years from now we will know whether these targets work as predicted. If they keep what they seem to promise, then they could be used for very interesting experiments.
Acknowledgment
This work has profited very much from discussions with members of the FILTEX and HERMES collaboration, in particular with J.v. d. Brand, M. Diiren, D. Fick, H.G. Gaul, G. Graw, W. Haeberli, J.R. Holt, E. Jaeschke, W. Korsch, W. Luck, Z. Moroz, H. Poth, B. Povh, R. Ransome, K. Rith, P. Schiemenz, T.-A. Shibata and K.
Zapfe.
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