Barber, D P

Bialowons, W

Camerini, U

Goetschel, H

Klanner, R

Lewin, H C

Lohrmann, E

Lomperski, M

Patel, P

Romanowski, T

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ParticleAccelerators, 1990, Vol. 32, pp. 173-178Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaA Polarimeter for HERAD. Barber, W. Bialowons, U. Camerini~ H. Gotschel!R. Klanner, E. Lohrmann! H.-Ch. Lewin, M. Lomperski;P. Patel~ T. Romanowski~Deutsches Elektronen-Synchrotron DESYNotkestrafle 85, 2000 Hamburg 52, GermanyAbstractThe electron-proton machine HERA is under construction at DESY inHamburg. The machine consists of two independent storage rings designedfor 30 GeV electrons and 820 GeV protons. The counter rotating beams cancollide head on in four interaction regions spaced uniformely around its 6.3 kmcircumference. At two of them HEP detectors are under construction: Theexperiment HI is situated in the North area and the experiment ZEUS in theSouth. In an ideal circular electron machine the spin of the particles becomespolarized transversely by the SokoIO'lJ- TernOl1 effect. The second generationHERA experiments will be done with longitudinally polarized electrons. Thisrequires the installation of spin rotators. But as a first step the build up of thetransverse spin polarization without rotators will be investigated and for thisexperiment a polarization monitor is necessary. This paper describes plans forthe first polarimeter for HERA, designed for measuring the transverse polar-ization. This monitor utilizes the Compton scattering of circularly polarizedlight: The number of back scattered photons is asymmetric with respect tothe machine plane when the target electrons are vertically polarized. Duringthe next stage of the commissioning of HERA, in August and September thisyear, it will be possible to make background measurements in preparation forthe installation of the complete polarimeter next year.IntroductionIn an electron storage ring the circulating particle beam may become polarizedtransversely by the Sokolov- Ternov effect [1]: In a flat storage ring the spin quan-tisation axes are aligned almost parallel to the bending field. However, synchrotronradiation can cause the spin to flip from up to down and vice versa. The two transi-tion rates are unequal and asa result the beam becomes spin polarized antiparallel• University of Wisconsin, USA and University of Bologna, Italyt Universitat Hamburg, Germany: University of Wisconsin, USA§McGill University, Canada1fOhio State University, USA[1615]/173174/[1616] D. BARBER ET AL.to the guide field; the spin orientation of positrons is parallel to the field. Thedegree of polarization increases in tilne according to(1)where P00 is the asymptotic equilibrium degree of polarization and r the polar-ization build up time. In the absence of depolarizing effects these parameters aregiven by8 1 n13 ""\15Poo == M == 92.4% and - == 3.53410-19 - -.'-5y3 T s Rp2where R is the Inean radius, p the bending radius and ,moc2 the beam energy.For future high energy experiments the polarization of the electrons could beused as an additional degree of freedom. For deeper investigation of the electro-weak interaction and to search for new phenomena, it would be very useful if theelectrons in the elec.tron-proton machine HERA could be longitudinally polarizedat the interaction points. For this reason it is intended to convert the vertical polar-ization in the arcs to longitudinal polarization at the interaction points with specialarrangements of vertical and horizontal bending magnets called spin rotators[2].In the presence of rotators the degree of polarization is reduced. But already ina flat machine various effects, like closed orbit distortions, could destroy the po-larization. Thus it is essential to investigate the built up of the polarization at anearly stage of the machine. As a development first step a monitor for measuringthe degree of polarization will be installed and its behaviour investigated.Various methods have been used to Ineasure the beam polarization in electronstorage rings. The polarimeter for HERA is based on the Compton scatteringof circularly polarized photons from a laser. If the electrons are polarized trans-versely, the angular distribution of the scattered photons has the form[3](2)whereP is the degree of polarization and ¢ is the azimuthal angle relative to thepolarization direction. The choice of the sign depends on the laser light helicity.From this formula we can see that the number of backscattered photons is aSYln-metric with respect to the Inachine plane when the electron beam is verticallypolarized and the incident laser light is circularly polarized.The Electron Storage Ring of HERAThe electron-proton machine HERA has been under construction since April6, 1984 and is scheduled to be completed at the end of next year. HERA consistsof a 6.3 km long circular tunnel housing a 820 GeV proton storage ring on topof a 30 GeV electron storage ring. Beams of both rings can collide head on atfour interaction points. In the first stage two experiments will be installed: Theexperiment HI is situated in the North area and the experiment ZEUS in theN$A POLARIMETER FOR HERA\\\\\\\\\" ./\..,...,.""'"..,..., ......\"\"""\""\\-1£' ~·s ZEUS. E[1617]1175Figure 1: Overview of HERA and the injection complex.experiment. HI is situated in the North area and the experiment ZEUS in theSouth (see Figure 1). The straight section in hall East is identical to the sectionscontaining these experiments. There, a first spin rotator will be installed forstudies on longitudinal polarization.The electron storage ring of HERA is complete. First storage trials were madelast year. During the second commissioning period, in August and September thisyear, the installed rf-power should make storage of 26 GeV electrons and positronspossible. Next year, after superconducting cavities have been installed, higherenergy will be possible.Parts of a polarimeter will be installed in octant West right at the end ofAugust. The outlet vacuunl chamber for the backscatterd photons and a showercounter are now available. With this equipment, measurement of backgrounds willbe possible.HERA has been described in other papers[4]. The status of HERA is presentedas an invited contribution to this conference. Some parameters are shown againin Table 1.Circumference L 6335.83m Polarization time TMean radius R == 2: 1008.38m for E == 30 GeV 25 minBending radius p 608.13m Rf power (in 1989) 7.6MWTable 1: Parameters of the HERA e- storage ring.176/[1618] D. BARBER ET AL.Layout of the HERA PolariIneterThe layout of the HERA polarimeter is shown in Figure 2. The principle issimilar to that of the polarimeter used at SPEAR[5], PETRA[6] and DORIS[7]:In Hall West a high power laser will be installed outside the shielding wall. Thelaser light enters the interlocked area of HERA through a pipe (see Figure 3) andwill be guided by mirrors to the interaction region in octant West right. The lightenters the machine vacuum system vertically through a viewing port. and will bereflected by a remote controlled vacuum mirror towards the electron beam. Theinteraction region is between two weak bending magnets. At the point oppositeto the entrance, the laser light leaves the vacuum and can be analysed. Thecrossing angle between the laser light and the electron beam is about 3.5 mrad.The electron beam and the backscattered photons are separated by a bendingmagnet. In order to make the synchrotron radiation spectrum soft, the electronphoton separator is a weak bending magnet with a radius of about 3220 m. Therate of synchrotron radiation photons is negligible above energies of 500 keY (nearlyno pair production). After a drift space an additional strong bending magnet witha radius of about 524 m increases the separation and at the end of this magnet thephotons leave the vacuum through a 0.5 mm thin aluminium window. Then theangular distribution with respect to the machine plane can be analyzed by a showercounter. The detector measures the centre of gravity of the shower generated by abackscattered photon and the asymmetry is deduced from the photon distribution.Laser System. Two laser systems are being considered. For the single photonmethod a laser with a relatively low peak power, but with a high repetition rateor duty cycle will be used. In this scheme, photons arrive singly at the detector.For the multi photon method a laser with high peak power, but low repetition ratewill be used. Here, thousands of photons arrive at the detector simultaneously in8: Oipol magnetQ: Quadrupol magnetf/mm50_=1.6mrad _ = IJ.5mrad WRFigure 2: Layout of the HERA polarimeter.A POLARIMETER FOR HERA [1619]/177Figure 3: Perspective view HERA Hall West.bursts. The advantage of the second system is that the instantaneous backscatter-ing rate is much higher so that backgrounds are insignificant. However,the lasersystem is more complicated than in the single photon scheme. The choice of asystem will be made after the backgound measurements.Vacuum Components. The two vacuum chambers of the polarimeter, the in-teraction region and the separation region with the outlet, are made of 4 mm thick~xtruded copper. The complete chambers consisting of cooling channels, beamchamber, pumping channel, flanges and beam monitors are brazed in a 15 m longstove inside a protecting gas atmosphere. This technique was developed for thestandard vacuum chambers of the electron storage ring. With this chamber 90 %of the synchrotron radiation is absorbed in the watercooled chamber walls. Inaddition the chambers are covered by lead shielding.Shower counter. A cross section of the shower c.ounter is shown in Figure 4.The detector consists mainly of a stack of lead framed tungsten plates, each 6.3 mmthick and scintillator plates, each 3 mm thick. The entrance window of the detec-tor is a 15 mm thick lead plate for shielding against the soft synchrotron radiation.A high energy photon entering the shower counter will be converted into a showerof charged particles which excite visible light in the scintillator plates. The scin-tillation photons are detected by four photomultipliers which are connected via178/[1620] D. BARBER ET AL.Cut A-AFigure 4: Cross section of the shower counter.wave length shifters to the scintillator plates. The scintillator plates are opticallyinsulated at the horizontal centre line and the centre of gravity of the showergenerated by a backscattered photon will be measured by comparing the photo-multiplier signals. The detector is mounted on a relnotely controlled table thatcan be moved both horizontally and vertically. With this table the shower countercan be precisly aligned with respect to the backscatt.ered photon beam. To shieldit against the "4 7T''' background, the detector is housed in a lead cover.SullllllaryThe electron storage ring of HERA has begun to operate and a polarization moni-tor is currently being installed. Background measurements will be made this year.The polarimeter will be completed next year. Then the electron energy will alsobe high enough, and T small enough, for measurements of polarization to begin.References[1] A. A. Sokolov and I. M. Ternov, On Polarization and Spin Effects in the Theory ofSynchrotron RtLdiation, Soviet Physics - Doklady, Vol. 8, No. 12 (June, 1964) 1203.[2] J. Buon and K. Steffen, HERA Variable Energy Mini Spin Rotator E3, DESY HERA85-09, February 1985.[3] F. W. Lipps and H. A. Tolhoek, Physica XX, 85-98, 395-405.[4] Gustav-Adolf Voss, The Hera-Project, Proceedings of the European Particle Accel-erator Conference, Rome, June 7-11,1988, Vol. 1, 7-10 and references there.[5] D. B. Gustavson et aI., A Backscattered Laser Polarimeter for e+ e- Storage Rings,Nuclear Instrunlents and Methods 165 (1979) 177-187.[6] Rudiger Schmidt, Aufbau und Test des Polarisationsmonitor an PETRA, DESY M-80/04, April 1980.[7] T. Linlberg, Eine genaue Bestimmv.ng der Ma.ss£: des Y Teilchens, DESY M-84/03,February 1984.

A Polarimeter For Hera

A Polarimeter For Hera

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