Polarized Jets And Storage Cell Targets
For Storage Rings
Erhard Steffens
Physikalisches Institut, University ofErlangen-Niirnberg
steffens@physik. uni-erlangen. de
Abstract. The state of the art of polarized gas targets for storage rings is reviewed. The storage
cell technique is inevitable for producing densities with sufficient luminosity for electron
scattering experiments. Different sources of polarized atoms for jets and storage cell targets are
compared. Remarkable progress has been achieved over the last 15 years, which has led to a vast
amount of experimental results with high statistics and good systematic precision. For hydrogen
or deuterium gas targets based on atomic beam sources and cold storage cells an areal density of
1014 atoms/cm2 with high polarization has been achieved. For a next generation experiment this
figure could be considerably enhanced.
I. INTRODUCTION
Targets discussed in this review are employed in spin-dependent scattering experiments on nucleon structure and on the interaction between nucleons. The most direct
approach to nucleon targets is hydrogen for the proton, and deuterium as weakly
bound proton-neutron system. Both hydrogen isotopes can be polarized by SternGerlach separation in inhomogeneous magnetic fields and subsequent rf transitions in
a very flexible manner. Due to its strong binding hydrogen atoms recombine at
elevated densities unlike 3He which can be compressed as polarized gas to high
pressure. Therefore hydrogen targets are ideally suited for storage rings with their high
stored current of several mA up to more than a hundred mA resulting in luminosities
of 1029 to several 1031/cm2s. Polarized gas targets in storage rings offer the following
distinct advantages:
• No dilution of polarization due to their isotopic purity unlike solid polarized
target material where a high contamination of background material is present,
e.g. about 6/7 of nitrogen nucleons and 1/7 of polarizeable protons inNHs.
• Detection of low-energy recoils enabled.
• High systematic precision due to rapid switching of the sign of Pz (and Pzz) on a
ms time scale if required.
• No background of unwanted reactions which might have a much higher cross
section, e.g. pion production in pp scattering at threshold (PINTEX) which
would be completely dominated by pions produced on heavier nuclei; the same
holds for deeply virtual Compton scattering (DVCS) on the nucleon as observed
at HERMES for the first time.
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. L Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
186
II. METHODS FOR POLARIZED GAS TARGETS
Beam Of Polarized Atoms: Jet Target
After pioneering work at the Stanford tandem accelerator in the 1970s [1], the first
polarized Jet target experiment in an electron ring was performed at the 2 GeV
VEPP-3 storage ring in Novosibirsk [2]. The target thickness achieved in 1985 was
2- 101 ^-atoms/cm2. A Jet target of similar density is presently employed at the EDDA
experiment at COSY (Julich) [3]. A hydrogen Jet of well known polarization will be
used at RHIC (BNL) in order to establish a polarization standard up to proton energies
of250GeV[4].
Whereas these targets are based on atomic beam sources (ABS) a new approach
studied at Michigan [5] makes use of hydrogen atoms cooled to < IK by means of
walls covered with superfluid He. In a strong magnetic field of several Tesla the
thermal energy Eth becomes small compared with the magnetic energy Emagn = ±JHB-B.
Then one electron spin state is trapped in the high field region, while the other state
with mj = +1/2 is extracted from the solenoid field and accelerated by the fringe field
to a fairly monochromatic atomic beam. After focusing by a parabolic mirror and a
large -bore sextupole magnet a dense target spot is produced. Here the dependence of
the target areal density t on the inverse velocity helps to enhance the density.
Polarized Atoms Injected Into A Storage Cell
There are two ways of injecting polarized atoms into a cell:
Ballistic Flow, i.e. from an atomic beam source for H or D atoms;
Flow driven by pressure gradient, i.e. from a laser-driven source for H, D or 3He
working with gas at a stagnation pressure above the one present in storage cells.
The storage cell has been proposed by W. Haeberli in 1965 [6], probably inspired by
the storage bulb of the hydrogen maser [7]. Applications as storage ring targets were
proposed by Schiiler [8], and Haeberli [9], in the early 80s. The basic idea is sketched
in Figure 1. A particle current It of polarized atoms is fed into the center of a T-shaped
storage cell, e.g. ballistically via its feed tube. Beam particles with current Ib circulate
along the axis of the straight beam tube. Target atoms diffuse from the center outwards
through all three tubular structures, resulting in a triangular density profile with central
volume density po which is given by po = It/Ctot, with Ctot being the total conductance
from the center outwards. The target areal density t is then given by
•
•
t = L . p 0 = L-It/Qot
(1)
The first test of a storage cell has been performed in 1980 by the Wisconsin group
[10]. H atoms from an atomic beam source were injected into a Drifilm-coated Pyrex
vessel. The estimated average number of wall bounces per atom was about 900. The
polarization was determined by the left-right asymmetry in elastic (a,p) -scattering. In
a weak magnetic field, the proton polarization of the upper two substates is Yz. After
subtracting a 50% background a proton polarization of P z = 0.43 ±0.07 was deduced
187
FIGURE 1. Schematic of a storage
cell target located in a vacuum chamber on axis of the stored beam. The
energetic beam of electrons or protons
pass the straight beam tube. Scattered
particles may leave the cell via its thin
walls and exit from the vacuum
chamber by means of an exit window
into the detector. A beam of atoms
from the source is injected into the cell
center via a feed tube. The atoms form
a triangular density distribution
I<at./s)
-L/2
+L/2
which is compatible with no depolarization. The areal density viewed by the Si
detector system was l.M012/cm2, which was more than a factor of 5 improvement
over free jets.
Design Of Storage Cells
Requirements: The cell walls must not limit the acceptance of the ring which
depends on the local machine functions. As t goes with the inverse total conductance
and for a round beam tube roughly with 1/r3, r being the inner tube radius, it is
important to provide small beam size at the target position. If ai are the rms beam radii
(i = x, y) then depending on the machine type the following condition for the half axis
of an elliptical beam tube might be adequate:
(i = x, y)
(2)
The cell length determines strongly the available target density t (see equation 1).
For vanishing conductance of the feed tube t scales with L2. On the other hand, L must
be chosen in accordance with the detector requirements which may limit the maximum
length to 250mm (PINTEX, [11, 12]) or 400mm (HERMES, [13, 14]).
The cell walls should be as thin as possible for the passage of scattered particles. In
case of an electron storage ring they have to be of metal in order to shield the bunch
field from the detector and to avoid rf heating of the target chamber. A suitable
coating like Teflon or Drifilm is applied for minimum depolarization and
recombination. By cell cooling the density is enhanced with (Tceii)"1/2. The minimum
temperature is determined by the onset of recombination below 60K.
Example: The PINTEX Cell. This experiment [11] runs at the IUCF cooler ring
with proton beams of up to about 450MeV. It is optimized for the detection of elastic
pp and pd scattering and pion production near threshold by Si strip detectors close to
the cell and tracking and scintillating detectors downstream the exit window[12]. Very
thin cell walls at room temperature are provided by suspending 450|Lig/cm2 Teflon
foils by means of a system of fins, as shown in Figure 2. The detector is designed such
that the symmetry of the cell is taken into account. A weak guide field in all three
directions x,y,z is applied by a system of orthogonal coils and can be switched rapidly
188
Teflon
Foil
Brace-
ABS
Feedtube
FIGURE 2. Schematic picture of
the PINTEX target cell [11],
consisting of four quadrants. The
250mm long beam tube of
quadratic
cross
section
lOxlOmm2 is formed by Teflon
foils suspended by four pairs of
fins fixed by braces. A polarized
H or D atomic beam is injected
via the feed tube. Unpolarized gas
can be admitted to the cell center
as well by means of a capillary.
Scattered particles may exit under
all azimuthal directions except
the horizontal and vertical plane.
in order to change the polarization components Px,y,z in the reaction system.
Example: The HERMES Cell: The cell [15] is designed for operation in the
27.5GeV HERA electron ring with very short bunches and peak currents of more than
100A. It consists of 75jam thick pure Al walls coated with Drifilm and has an elliptical
cross section of 21.0x8.9mm2. The cell is suspended by cooling rails cooled with cold
He gas to temperatures of 100K and below. Throughout the experiment the electron
beam is surrounded by conducting walls of slowly varying cross section for
suppressing the excitation of wake fields. A system of tungsten collimators upstream
of the cell serves to protect the cell walls from Synchrotron radiation. With the help of
the involved target diagnostics even small changes of the cell wall properties can be
detected. After operating with beam clear signs of detoriation of the Drifilm coating
are visible. It has turned out that under normal operating conditions a layer of frozen
water is formed which serves as renewable surface with excellent properties [14, 15].
The water is supplied via the atomic beam by the ABS dissociator [16] which is run
with H2 or D2 and a small oxygen admixture for optimum dissociation fraction. C>2 is
converted in the discharge to water.
III. SOURCES OF POLARIZED GAS
An overview of operating targets and targets under design or construction is given in
Table 1. Target types are labeled "Source-Target" with ABS = atomic beam source,
TABLE 1. Comparison of operating targets and targets under design or construction.
Experiment
Target Gas
PINTEX / IUCF
EDDA / COSY
DEUTERON / VEPP-3
HERMES / HERA
ANKE / COSY
Polarized Jet for RHIC
BLAST / Bates
"
"
"
"
H,D
H
D
H,D
D, H
H
H,D
H
3
He
Type
ABS - SC
ABS- J
ABS - SC
ABS - SC
ABS - SC
ABS- J
ABS - SC
LDS - SC
OP -SC
189
Beam
Part.
P
P
e
e
P
P
e
e
e
Status
terminated (2002)
operational
operational
operational
under construction
under design
commissioning
under development
proposed
LDS = laser-driven source, OP = optically pumped source, and J = jet, SC = storage
cell. In the following, three source types are discussed.
1. Atomic Beam Source: A modern ABS starts with the formation of a cold
hydrogen atomic beam from a dissociator with cold Al nozzle and powerful
differential pumping system, based on turbomolecular and cryogenic pumps of total
pumping speed of about 104l/s. Spin-dependent focusing is provided by high-field
permanent sextupole magnets with pole tip fields of up to 1.6T [17]. Nuclear
polarization Pz (Pzz) is produced by a system of rf-transitions with nearly 100%
efficiency. For H and D in two substates beams with polarization close to the
maximum possible values can be produced and switched rapidly.
The production of cold atomic beams of high brilliance has been studied over the
last 15 years at various labs incl. ANL, Bonn, ETH Zurich, CERN, Dubna,
Novosibirsk, and by a Heidelberg-Marburg-Munich-Wisconsin collaboration. The
particle current accepted by a standard compression tube of 10mm diameter and
100mm length which can be measured to a few % error, went up by a factor of 3. The
sources exhibit very low contamination in the atomic beam important for cryogenic
cells, and they are extremely reliable as proven by the HERMES ABS which has been
operated over the last 6 years for most of the calendar time with very high availability.
The particle flow rate of H beams in two substates seems to saturate at values below
1017/s. Systematic studies of beam formation are performed at DESY by the HERMES
target group. The recent work on the target test bench was described at this
symposium by Nass [16]. The measured velocity distributions and intensities as
function of gas flow and nozzle temperature are reproduced by DSMC simulations. A
recent attempt to increase the beam intensity by applying a hollow dense carrier jet
failed due to the strong admixture of carrier gas to the central beam which can be
reproduced by recent simulations [16].
2. Ultracold Source: The status of the Michigan Jet was reviewed at PST01 by
Luppov [5]. The improved version comprises a 12T solenoid, lOOmW of cooling
power at 300mK for the separation cell with parabolic mirror, a large-aperture rf
transition and sextupole magnet, and a powerful cryoabsorber as beam catcher. By
means of a slit-type compression tube of 11.0-1.4mm2 a maximum intensity of
2.2-1015H/s in two substates has been detected, corresponding to an areal density of
l.M012H/cm2.
3. Laser-Driven Source: The basic idea is to employ the high number of polarized
photons from a laser to polarize hydrogen. As there are no UV-lasers available for
direct optical pumping of ground-state hydrogen atoms an indirect approach is to
optically pump a small admixture of K atoms and to transfer their polarization by spin
exchange collisions to the hydrogen atoms [18]. The arrangement is shown
schematically in Figure 3. The glass cell with Drifilm coating is kept between 150 and
200C in order to avoid condensation of potassium. The storage cell must be operated
at a similar temperature, which is about five times higher than cryogenic cells used
with AB sources. The anticipated flow rates and target densities are well beyond ABS
figures but the feasibility of such targets has not been demonstrated yet.
The main difficulty is to maintain a high dissociation fraction a at the exit of the
pumping cell and in the target cell, in particular its long-term performance. By the
190
FIGURE 3. Schematic picture of
the Laser-Driven Source [19].
Atomic hydrogen is injected into
the heated spin-exchange cell
where the interaction with
optically pumped K atoms takes
place. Circular polarized photons
at 770nm from a Ti-Sapphire
laser are shined in parallel to the
holding field. The gas then flows
into the storage cell target which
is kept at a similar temperature as
the cell in order to avoid
condensation of potassium.
presence of molecules the polarization of atoms is diluted. Assuming vanishing
nuclear polarization of molecules the quality factor QF is given by
t being the areal target density, Pavthe average nuclear polarization of target particles,
and Pat the polarization of atoms.
The LDS development started at Argonne in 1984 [20] and was continued by work at
Erlangen, Illinois, and recently MIT [19]. Probably the most systematic approach has
been chosen at Erlangen [21] by studying the basic processes using an involved
diagnostic system, including
• a Breit-Rabi polarimeter to measure electron and nuclear polarization of atoms;
• a Faraday monitor to measure K density and polarization;
• an optical monitor to measure a in the glass tube of the dissociator by comparing
intensities from Balmer and molecule spectra;
• the measurement of a at the source exit via intensity modulations in the BRP.
The results are used to get a better understanding of the various processes taking place
in this rather aggressive environment. An important finding was that spin-temperature
equilibrium is attained in such sources [22] which enables to generate in addition to
electron polarization nuclear polarization as well.
A test experiment on the LDS target at the IUCF cooler ring has been performed by
the CE66 Argonne-Erlangen-Illinois-Colorado collaboration in 1996-99. The nuclear
polarization of the hydrogen and deuterium targets was determined by elastic
scattering of polarized protons. The results were presented atPST99 [23]. A common
experience of all targets to date is that the atomic fraction a decreases considerably
when the K ampoule is heated to the operating temperature. Clearly the surface
detoriation in the presence of the alkali needed for optical pumping seems to be the
most serious limitation for a LDS target [23]. The a values measured in the course of
the test experiment were around 0.4 and the electron polarization around 0.5. The
average nuclear polarization was up to about 0.15 for hydrogen, and about 0.10 for
deuterium. -This result may be compared with typical parameters of the HERMES
target by calculating the quality factor QF according to equation 3.
191
The resulting QF's are:
= 0.12 2 x4-10 1 7cm 2 = 6-1012/cm'
QF (CE66)
(4)
QF (HERMES) = 0.852x 1- 1014/cm2 = 75- 1012/cm2
Despite the much higher flow rate and four times higher density the LDS target turns
out to be more than an order of magnitude lower in quality factor. Further progress is
ultimately linked to improved wall properties with a considerably higher a.
Target Polarimetry
There are two scenarios of determining the target polarization required for evaluating
spin-dependent quantities:
• Measurement of absolute target polarization by scattering of beam particles with
known analyzing power and sufficient scattering rate. In this case monitoring of
the electron polarization Pe for setting up and control is sufficient.
• In case of no useful reaction with beam particles due to insufficient rate like in
deep-inelastic electron scattering an independent absolute target polarimeter is
required.
The 2nd approach is realized in case of the HERMES target for which the so-called
Breit-Rabi polarimeter (BRP) has been developed [24]. The diagnostic system is
shown in Figure 4.
sexp, magn.syst.
TGA
detector (QMS)
BRP
Fig. 4: The HERMES target diagnostics [24]. The sample beam from
the storage cell formed by the sample
tube
enters
the
Breit-Rabi
polarimeter (BRP) where its atomic
substate population is determined to
0.01 in precision within about Imin.
By means of the extension tube a
flow of target gas is directed to the
target gas analyzer (TGA) under an
angle of 7° to the BRP axis where the
atomic fraction a is measured.
The key problem is to relate the target parameters as seen by the high energy beam to
those of the sample of target gas extracted in the cell center. These so-called Sampling
Corrections are obtained from simulations which are constrained by the result of
measurements. In this way total errors 8P/P of 3-4% have been achieved [14]. By
measuring substate populations as function of the guide field one is - in particular at
low field - sensitive to spin exchange between nuclear and electron polarization. Their
rate is density dependent. Knowing the spin exchange cross section the density can be
extracted.
IV. PERFORMANCE OF OPERATIONAL TARGETS
In Table 1 the operational targets including the recently terminated PINTEX target
[12] are listed. The EDDA target at COSY (Julich) is designed as Jet target in a single
192
substate [25] and t = 2-1011 H/cm2are obtained. For the test of time-reversal invariance
(TRIG) the addition of a cell is in progress in order to increase t to about 1013/cm2. The PINTEX target [11,12] has been operated from 1993 to summer 2002 with
protons of energy up to 450MeV and has enabled the collection of a vaste amount of
pp and pd scattering data. The target cell with extremely thin Teflon walls has already
been described in section II (see Figure 2). In 2000, the target was upgraded to
deuterium and successfully employed for experiments. As a surprise a considerable
reduction of Pzz to about half of the expected value was observed [26] which could be
traced back to spin exchange at weak guide field. - The DEUTERQN target at
Novosibirsk reaches back to the first deuterium jet target in an electron ring operated
in the mid-80s [2] which was complemented by the first storage cell in a storage ring
in 1988. Attempts of an Argonne-Novosibirsk collaboration to implement a LDS
deuterium target were unsuccessful and terminated. The target now comprises a
cryogenic ABS with superconducting sextupoles and a storage cell at nitrogen
temperature [27]. A record ABS intensity of 8-1016 D/s has been achieved. Future
plans include to employ ee scattering on polarized target electrons for the
measurement of beam polarization. - The HERMES target [14] has been operated
with longitudinal polarized hydrogen in 1996-97 and with longitudinal polarized
deuterium in 1998-2000. It is operated more than 8 months per year plus setting up
and testing. Since 2001 the target runs with transverse polarized hydrogen for which a
new guide field magnet with high uniformity has been developed [28] required to
inhibit a new class of densely-spaced depolarizing resonances.
V. STATUS OF FUTURE FACILITIES
As shown in Table 1, there are three new facilities where polarized gas targets will be
applied, two at proton rings and one at an electron ring.
1. The ANKE target [29, 30] at COSY: The state of the art ABS for polarized H and D
atoms is operational and has produced up to 7-1016 H/s into the compression tube. A
Lamb shift polarimeter is used to optimize the system of rf transitions. The target
chamber with storage cell and Si strip detector is under construction and will be
installed at the beginning of 2003.
2. The Jet polarimeter [3 L 32] for RHIC: A polarimeter for RHIC with high statistics
based on CNI is relative only. The idea is to relate elastic scattering of unpolarized
RHIC protons on a hydrogen Jet of well known polarization to the same reaction
between the polarized RHIC beam and an unpolarized (spin averaged) Jet target. The
Jet will be produced by an ABS and the polarization measured by a Breit-Rabi-type
polarimeter. For the required precision of 3% of the target polarization the molecular
background at the interaction point has to be measured.
3. The BLAST target [33] at Bates: The BLAST detector is based on a 4n toroid
spectrometer with the target cell at the center. Apart from the planned OP source for
3
He and a LDS target source [19], an ABS target is foreseen. This target utilizes the
former AmPS ABS which is modified in order to meet the requirements imposed by
the spectrometer, e.g. by placing the turbopumps outside the coil system. Other
193
sensitive components are heavily shielded against magnetic fields. At present the
target is in the commissioning phase [33].
VI. CONCLUSIONS AND OUTLOOK
The status of gas targets for storage rings may be summarized as follows.
• These targets represent a mature technology which is well understood and they
have enabled new Spin experiments, as single users in medium-energy storage
rings like the IUCF Cooler, VEPP-3 and COSY, and in future at the Bates south
hall ring, and as parallel user in a high-energy storage ring like HERA.
• A characteristic feature is the extremely low background which enables to study
reactions with low cross sections, e.g. pion production at threshold, and to utilize
open spectrometers as for PINTEX, HERMES, or BLAST where the full angular
range is measured at once.
• New experiments based on state of the art gas targets should lead to a much
improved performance. BLAST is a first example. A similar proposal exists for
COSY, the so-called TOROID experiment to study heavy meson production at
threshold [34].
In the outlook we might speculate about possible improvements. The presently
available technology for H & D targets is the atomic beam source with (cold) storage
cell. The ABS intensity seems to be limited to about 1017 atoms/s in 2 substates. By
increasing the cell length considerably the target densities could be strongly
improved. This is particularly important for electron rings where luminosity is a
crucial factor. For a long cell the total conductance (see equation 1) is dominated by
the feed tube and might be as low as 51/s for a cold cell which results in a central
density of p0 = 2-1013/cm3. With a 1m long cell areal densities in the order of t =
1015/cm2 are conceivable. This would lead for an improved HERMES experiment as
a single user of the HERA electron ring to integrated luminosities of up to 3fb~1 per
year, which is about 20x the luminosity of the "Golden Year" 2000 of HERMES! We
should not neglect that such a long, high density target puts stringent requirements on
the design of the tracker system. Also there will be a much higher spin exchange rate
which might inhibit a precise measurement of the target polarization by means of a
sampling BRP. On the other hand at such luminosities it appears feasible to
normalize the product of beam and target polarization to the inclusive DIS
asymmetry which is well known.
ACKNOWLEDGMENTS
I would like to express my sincere gratitude to W. Haeberli for the pleasant and fruitful collaboration
on polarized gas targets over the last 15 years. Valuable help in the preparation of this talk by D.
Eversheim, M. Henoch, H. Kolster, P. Lenisa, A. Nass, F. Rathmann, D. Toporkov, J. Wilbert, T.
Wise and A. Zelenski is gratefully acknowledged. My thanks are due to the Organizers, in particular
to Th. Roser and Y. Makdisi, for inviting me to give this talk, and to the Bundesministerium fur
Bildung und Forschung, Germany, for financial support.
194
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