Hadron and photon beams for bubble chambers

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Nuclear Physics B (Proc. Suppl.) 36 (1994) 323-334 North-Holland

Hadron and Photon Beams for Bubble Chambers J. Sandweiss Yale University, Physics ~ t ,

New Haven, CT 06520, USA

1. S p e c i a l R e q u i r e m e n t s f o r B u b b l e C h a m ber Beams The key feature of bubble chamber operation that affects the design of beams is the fact that the chambers cannot be triggered. As a result it is important that every particle entering the chamber during its sensitive time be a desired particle. The importance of each individual particle is further raised by the need to keep the total number of particles entering the chamber per sensitive time (i.e. per picture) below some maximum permitted by obscuration of one track by another. In practical situations this corresponded to a flux limit of the order of 20 tracks per picture. In the case of photon beams, the photons, of course, leave no track but the obscuration limit is due to the e + - e - pairs produced in the bubble chamber liquid or in the beam window just upstream of the sensitive volume. For charged particle beams, the need to expose the chamber primarily to the desired particles led to the development of separated beams, containing only the wanted particles. For photon beams the limit on the total pair flux led to the development of mor~ochromatic beams. There are other types of beams where important work was done, for example K ° beams, but as these where not a major part of bubble chamber history, this review will concentrate on separated beams and monochromatic photon beams. 2. Electrostatically Separated B e a m s

In the early 1950's ideas on separated beams involved several approaches. These included separation based on the dependence of the energy loss rate on velocity, and the use of electrostatic deflection. The ultimate "winner" of these different methods was the electrostatic separation using parallel plate separators. It should be noted, 0920-5632/94/$07.00 © 1994 SSDI 0920-5632(94)00545-7

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however, that early schemes proposed by J. Murray involved coaxial separator designs. The basic principle of electrostatic separation is illustrated in figure 1. The transverse m o m e n t u m imparted by the electrostatic field is APt given by

APt = f F, dt = eEUc# The electric deflection is thus velocity dependent while a magnetic deflection is m o m e n t u m dependent. The combination of magnetic m o m e n tum selection and electric deflection in an electrostatic separator thus comprises a mass selection. Of course, care must be paid to m a n y details to use this method in a practical design. A m o n g the effects which must be dealt with are the aberrations of the magnetic focusing and deflecting elements, collimation and slit scattering, and effects due to the finite m o m e n t u m band of the transmitted beam (so called chromatic effects).

Elsevier Science B.V. All rights reserved.

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The first operational electrostatically separated beam was the 1.17 BeV/c K - beam for the 15 inch bubble chamber at the Bevatron [1]. Electrostatic separators and beams were vigorously developed at the CERN PS, the Berkeley Bevatron, the Brookhaven AGS, and the Argonne National Laboratory ZGS. Beams were built with K ± up to 6 GeV/c, zr±, and p± up to ~ 7 GeV/c. References [2], [3], [4], and [5] describe some of the major systems which were brought into operation during the first half of the 1960's. A dramatic example of the virtue of separated beams is shown in figure 2 which shows the response of the Berkeley 72 inch chamber exposed to the beam described in reference [5]. The three views show the beam through the bubble chamber for a) no separation, b) one separator operating and c) both separators operating. In all three cases the flux of primary beam on target is the same. The layout of the beam of reference [5] is shown in figure 3. The technology of high voltage electrostatic separators was developed at Berkeley, Brookhaven, and CERN and are noted in the references cited. A typical electrostatic separator construction as used at the Bevatron and similar to that used at the AGS is shown in figure 4. These devices used cathodes made of conducting glass. The basic idea was that with a finite resistivity, sparking would be quenched. The conductivity must, of course, be high enough so that the electric field has the correct shape to an adequate accuracy. The proper conductivity of the glass was obtained by varying the temperature until the conductivity was high enough so that the electrode surface was indeed an equipotential plane. Thus, these separators were built as "thermos bottles". T h a t is, with a double wall so that excessive heat loss to the outside did not occur. At CERN, separator construction did not use hot glass cathodes. The CERN physicists also found that in their designs it was important to avoid both a (large) electric field and a magnetic field on the high voltage bushing. Figure 5 shows the design of the separator of reference [4]. In this design the magnetic fields which "straighten out" the trajectories are located in lumped magnets at

each end of the separator. With these techniques, many beams were built ranging from low energy stopping kson beams to high energy beams of kaons up to 6 G e V / c and antiprotons to about 8 GeV/c. Vigorous programs were pursued at these facilities and much of the "underpinning" of the physics we know today was obtained from these studies. As a final example of the electrostatic beam technique, figure 6 shows the layout of the separated beam which produced the 6 GeV/c K - used in the exposure of the BNL 80 inch chamber in which the first example of the Omega Minus hyperon was observed [6]. This design is an example of a "chromatically corrected" beam. Notice the sextupoles which are placed near dispersive horizontal foci. Their role is to apply s momentum dependent correction to the vertical focusing and thus for the vertical dimension to obtain a near achromatic image. 3. R a d i o F r e q u e n c y S e p a r a t e d B e a m s Following on the successful research program using electrostatically separated beams, attention naturally turned to higher energies. The electrostatic method essentially utilized the different transit times for the (momentum selected) different particle species through the separator. The direct extension of the electrostatic method to higher energies would thus lead to impractical lengths for the separators. The nature of the problem is easily seen by considering the relativistic formula for the deviation from light velocity, A f t : 1 -- ( u / c ) , a s the energy, E, of a particle of mass m increases. m 2

Aft _~ 2E 2 The basic idea of the Radio Frequency (RF) separated beam is to pass a momentum selected beam containing wanted and unwanted particles through two separated, RF, deflecting structures. The drift time difference between the structures can be made to be a significant fraction of the RF period so that the action of the two deflectors will be different for the different components of the beam.

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The above scheme functions to separate two beam components for an unbunched beam. If the primary beam from the accelerator is bunched at the separator frequency, the same effect can be achieved with a single cavity. In practice the use of bunched primary beams for this purpose has been limited to electron linear accelerator beams. If there are two unwanted contaminants in the beam (unbunched), the separation with two cavities will only work at special energies for which the phase shifts of the two contaminants, between deflectors, differ by an integer multiple of 2~. If a continuous range of operation is desired, three deflectors must be used. It should also be mentioned that an alternative type of RF separation was proposed by Murray [10] which used two separated microwave structures operating in the accelerating mode to change the energies of two species of beam particles. Separation would then be achieved by a "post" momentum analysis. This scheme was analyzed to be less efficient than that based on transverse deflection and was never realized in practice. With drift lengths of the order of several 10's of meters and particle momenta in the 10-20 GeV/c range, it turns out that the S band frequency range is a suitable one. Because of the well developed technology including power sources, all RF separated beam designs have used the SLAC frequency (2856 MHz). The key to this is, of course, to have a structure which can support an RF field capable of imparting a useful transverse deflection to the beam passing through it. In the early 1950's there was some confusion about whether or not a cylindrically symmetric structure could support a deflecting wave which could impart a transverse deflection to an axially directed relativistic particle. The Panofsky Wenzel theorem [7] showed that all T M waves gave zero deflection and T E waves gave a transverse impulse proportional to 1 -/32. However, when/3 -- 1 the T E and TM modes are not independent and there does indeed exist another, so called hybrid or deflecting, mode. This mode can impart a transverse deflection to a relativistic particle, a clarification that was first published explicitly by Bell, Bramham, and Mon-

Figure 7. Brian Montague testing an early prototype of the CERN RF deflector structure, ca 1962.

tague [8], although it had been suspected earlier. Panofsky [9] had proposed an RF separator essentially along modern lines in an internal Stanford report. Also, the "pulse shortening" effect observed in the early operation of the SLAC linac was suspected of having its origin in the excitation of the deflecting mode. Figure 7 shows a photograph of Brian Montague at a test station for an early prototype of the CERN separator structure. The deflecting structures look like disc loaded waveguides with roughly similar dimensions. They have been built to handle peak power of _~ 20 Mw, leading to a transverse impulse for a 3 m length of ~_ 20 MeV/c. The schematic structure of a simple RF separated beam is shown in figure 8 and the phase space behaviour for such a separated beam is shown in figure 9 for the more realistic (but still idealized) case of a beam with finite emittance. The first suggestions for an RF separated beam at CERN were made by Schnell [11] in an interhal CERN report. These ideas were developed at CERN and at Brookhaven and resulted in succesful RF separated beams at those laboratories by 1965 and 1966 [12], [13]. Typical beam purities were greater than 95%. A useful and interesting paper on all aspects of the RF separation tech-

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nique is given by Montague [14]. As an example of such a beam, the schematic layout with characteristic optical rays for the BNL RF separated beam is shown in figure 10. The highest energy RF separated beam was constructed by a CERN IHEP (Serpukov, USSR) team for the Saclay bubble chamber "Mirabelle" which was moved to the 70 GeV proton synchrotron at Serpukov [15]. This beam used a three cavity design and operated with kaons from 17 G e V / c to 40 G e V / c and with antiprotons and positive pions up to about 50 GeV/c. Typical beam composition (at 34 GeV/e) was K + ,,~ 98%, background ,v 2%. 4. M o n o c h r o m a t i c

Photon Beams

As mentioned above, the limiting factor for the flux of photons which can be passed through a chamber is the obscuration due to associated pairs produced in the liquid or in the material directly upstream of the liquid. For this reason it is desireable to limit the photon beam energy spread to that (narrow) range which is under study in the experiment at hand. The method which developed as the successful approach to producing such monochromatic photon beams was the method of Compton back scattering of laser photons from the electrons of a monochromatic electron beam. Early suggestions of this approach were given in references [16], [17],[18], and [19]. A design for the 82 inch chamber at SLAC was proposed in 1967 by Murray and Klein [201. The basic idea is most readily understood by considering the limiting case in which the incident photon energy, hwi, is negligible compared to the electron rest energy, m e c 2, and the scattering is directly backwards (0 = 180°). In that ease it is easy to see that the scattered photon energy, /i~j, is given by hw! --= 4-r~hwi where 7 is the Lorents factor ( E e / m e c 2) of the electron beam. Applying this, for example, to a 20 GeV electron beam and an incident photon of 1.8 ev yields a backscattered photon energy of 10 GeV. In a real situation, of course, one must

consider that the electron mass is finite and that there is a range of angles included in the backscattered beam. Figure 11 shows the layout of the beam built for the 82 inch chamber at SLAC and figure 12 shows the photon spectrum obtained with that beam for two electron beam energies (12 GeV and 16 GeV). The advantage of the backscattered beam is particularly striking when one recalls that a brehmstrahlung beam would have a photon spectrum peaked at low energy. The highest energy monochromatic photon beam made in this way was the beam used for study of the photoproduction of charmed particles which was carried out with the SLAC hybrid bubble chamber [21]. Figure 13 shows the layout of that beam which used a 30 GeV electron beam and a frequency doubled Ruby laser. Figure 14 shows the photon energy spectrum at the chamber. The successful operation of this beam together with the high resolution cameras utilized at the hybrid chamber for this experiment gave many beautiful examples of charmed particle production and decay and helped to establish the now accepted basic understanding of these particles. 5. C o n c l u s i o n s

The bubble chamber technique was beautifully suited to the physics questions of its time. It inspired many ingenious and complex beam systems to most fully exploit the chambers. Because the beam systems were large and complex, they together with the other complex systems required to operate and use bubble chambers, gave rise to a number of worries about the technique. It is interesting to recall some of the "complaints" at that time. How can one work in such large groups as are required to utilize the bubble chamber technique? Are we training technical physicists who are too narrowly specialiEed to move on to new methods as they are required? It is clear now that the field,indeed, survived these challenges and it gives hope for the future progress in the face of quite similar concerns raised about the present style in our field.

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Figure 14. The photon spectrum at the SLAC hybrid bubble chamber. The bubble chamber beam developments pioneered the careful, professional approach to beam design. It "lives on" in the design of the beams for todays fixed target experiments and in the design of collider interaction regions. Finally, those of us who were involved with the activity will recall how exciting and how much fun it was! REFERENCES

1. P. Eberhard, M. L. Good, and H. K. Ticho Rev. of Sci. Instr. 31,1054 (1960) 2. C.A. Ramm, Proc. International Conference for High Energy Physics 1960 3. W . J . Fickinger, J. Sandweiss, and J. Sanford, "The Cosmotron Parallel Plate Velocity Spectrometer", Brookhaven National Laboratory Internal Report (1961) 4. C. Germain and R. Tinguely, Nuclear Instruments and Methods 20, 1963 5. J. Button-Shafer, G. R. Kalbfieisch, D. H. Miller, J. Kirz, C. G. Wohl, J. R. Hubbard, D. O. Hughes, H. K. Ticho, and D. H. Stork, University of California Radiation Labora-

tory internal Report UCRL 17018 6. I. Skillicorn and M. S. Webster, "Status Report on a High Energy Separated Beam for the 80 in Chamber", BNL Bubble Chamber Group Internal Report, H-10. 7. W . K . H . Panofsky and W. A. Wenzel, Rev. of Sci. Instr. 27, 967 (1956) 8. M. Bell, P. Bramham, and B. W. Montague, Nature 198, 277 (1963) 9. W . K . H . Panofsky, Stanford Internal Report HEPL 82 10. J. J. Murray Nucl. Instr. and Methods, 20, 26, (1963) 11. W. Schnell, CERN Internal Report 61-5 (1961) 12. P. Bramham, R. D. Fortune, E. Keil, H. Lengeler, B. W. Montague and W. W. Neale, Phys. Lett. 15,290 (1965) 13. H. W. J. Foelsche, H. Hahn, H. J. Halama, J. Lach, T. Ludlam, and J. Sandweiss, Rev. of Sci. Instr. 38, 879, (1967) 14. B. W. Montague, Prog. Nuc. Tech. _3, (1966) 15. PH. Bernaxd, J. S. Chodirev, N. A. Galjaev, A. Grant, V. I. Kotov, P. LaJeyras, H. Lengeler, B. Marechal, J. C. Prelaz, A. A.

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Prillepin, B. V. Prossin, and V. E. Zelenin, Nuclear Instruments and Methods 113, 295, (1973) 16. R. H. Milburn, Phys. Rev. Left. 10, 75 (1963) 17. F. R. Arutyunian, I. I. Goldman and V. A. Trumanian, Zh. Ekspcrim. Teiv. Fi~..45, 312

(1963) 18. F. R. Arutyunian, I. I. Go]dman and V. A. Tumanian, Soviet Physics, J E T P 18, 218 (1964) 19. F. R. Arutynian and V, A. Tumanian, Phys. Lett. 4, 176 (1963) 20. J. J. Murray and P. R. Klein S L A C Report SLAC-TN-67-19, June (1967) 21. Private communication, J. Ballam