The HERA-B ring imaging Cherenkov detector

Nuclear Instruments and Methods in Physics Research A 408 (1998) 191—198

The HERA-B ring imaging Cherenkov detector J.L. Rosen* Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA

For the HERA-B RICH Group1 Abstract The HERA-B RICH system is designed and prototyped. It will be fully assembled by the end of 1998. It will constitute an efficient and reliable means for charged particle identification over the momentum range 4—80 GeV/c. Its principal role will be to provide B0(BM 0) flavor tagging via kaon-pion separation. An overall description of the design features is provided. The principal innovation is the design of the focal plane photon detector. This employs Hamamatsu R5900 multianode photomultipliers exclusively. Each PMT is outfitted with a specially designed UVT acrylic lens telescope system for light collection purposes. ( 1998 Elsevier Science B.V. All rights reserved.

1. Introduction A comprehensive and updated HERA-B overview has already been presented by Thomas Lohse at this conference. Table 1 summarizes a few essential facts. Clearly the bbM production cross section is more than three orders of magnitude smaller than at a hadron collider. But there are partially compensating advantages to working so much closer to the threshold and having the motion of the CM strongly boosted forward. The production is well centered within our acceptance. We plan to record

* E-mail: [email protected]. 1 S. Korpar, P. Krizan, A. Stanovnik, M. Staric, (J. Stefan Institute and University of Ljubljana), R. Eckmann, J. McGill, R. Schwitters (University of Texas at Austin), D. Broemmelsiek, P. Maas, J. Rosen (Northwestern University), K. Lau, J. Pyrlik (University of Houston), M. Ispirian, S. Karabekian (DESY), I. Arino, M. Chmeissani, Ll. Garrido, R. Miquel (University of Barcelona), W. Schmidt-Parzefall, T. Oest (University of Hamburg).

the physics with very high acceptance, superior proper time resolution, momentum resolution, high-quality lepton detection and high flavor tagging efficiency. The last point is the main thrust of this presentation. It may well be that forward hadron collider systems coming into existence in the next decade will incorporate some of our design concepts and experiences into higher yield studies. One of the HERA-B principal tools for efficient flavor tagging, and ancillary spectroscopic purposes as well, is a large acceptance gaseous RICH system operated at near STP. Fig. 1 displays a side view of the RICH system. The fundamental vessel is a large welded steel structure outfitted with large area relatively thin entrance and exit windows. These windows are not expected to sustain differential pressures in excess of 10 mbar (always outward!). All flanges and light viewing ports must be gas and light tight. The RICH system occupies 3.5 m of space along the beam axis beginning at 8.5 m from the target. It is downstream from the fine resolution tracking systems and upstream of

0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 3 1 8 - 0

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Table 1 Some parameters of the HERA-B experiment Proton beam energy 820 GeV E 40 GeV CM c 21 CM (c~1"50 mrad and corresponds to 90° in the CM) Sp T +c (0.7 GeV)+14 GeV ,!0/ CM Spectrometer angular acceptance $160 mrad (vertical) $250 mrad (horizontal) (10 mrad occluded by beam pipe The acceptance +3 units of rapidity. The rapidity range of bbM production is41 unit of rapidity. Hence the acceptance is quite large and efficient. :B dz Crossing rate Interaction rate bbM pairs/s

+0.7 GeV 10 MHz 30—40 MHz +30

an array of pad chambers which are then followed by the electromagnetic shower detector. It is not anticipated that the accumulation of material constituting the RICH gas, windows, mirror and supports will impact severely on the shower system. Some radiation conversion (bremsstrahlung or gamma ray conversion) will take place, but energy resolution and shower cluster shapes will be little affected. The effect on the downstream muon system should be completely negligible. The impact of the RICH system on the pad detectors and their performance as key elements in the development of high transverse momentum hadron triggers is a much more complicated and worrisome matter. It will be under study for quite some time and will be discussed no further here. We assume that the reader is familiar with the basic idea of the focusing gaseous RICH system. The radiator region is terminated by a large multipaneled spherical mirror 7 mm thick hexagonal panels). The center of curvature (R) coincides with the target source. Cones of Cherenkov light are emitted and after reflection converge to sharp rings at the focal surface — itself a concentric sphere of half radius ( f"R/2). Now our fundamental mirror system is actually split into upper and lower halves, each half tilted by 9° with respect to the beam axis. The ring patterns are consequently deviated either up or down by 18°. The spherical aberration produced by the 9° tilt is quite minor in its conse-

Fig. 1. Cross-sectional view of the RICH detector. The stainless-steel sealed vessel (100 m3 volume) is located in the region 8.5—12.0 m downstream of the target region. The fundamental ring forming spherical mirror (R"11.5 m) is shown on the left. It is formed from 80 discrete mirror tiles. There is a horizontal split — at the median plane the upper and lower sections are effectively tipped by $9". A second set of multipaneled planar mirrors directs the Cherenkov light to matching sets of focal plane photon detectors.

quences. In effect, the mirror curvature in the tilt plane (vertical) is increased relative to the horizontal plane. The distance to the ideal focal surface is foreshortened in the vertical plane by approximately 6 cm (out of f"570 cm) and this has negligible resolution consequences. Other factors affecting the resolution of the RICH are listed in Table 2. For structural convenience a second planar mirror system is introduced into the twin RICH detector halves. This places the photon detection stations well above and below the beam line ($3 m). At these large angles, the density of energetic radiation directly striking the photon detector

J.L. Rosen/Nucl. Instr. and Meth. in Phys. Res. A 408 (1998) 191—198 Table 2 Factors determining RICH velocity resolution Cell size — for square cells, p"(width)/J12 Mirror quality and alignment accuracy Spherical aberration (negligible) Chromatic aberration (small) Track reconstruction and momentum accuracy Our chosen cell size is 9 mm. We anticipate that our working p for forward hemispheric angles will be (2.5—3.0) mm.

is low and the detector is largely shadowed by the magnet pole faces. Further the mirror reflection path of the Cherenkov photons is 17 ns longer than that of in-time background. There is little doubt that the radiation background in the RICH will be overwhelmingly Cherenkov light itself. Restated, an event with a hundred or more intersecting rings does not need any other background sources! Gamma ray conversions in the beam pipe promise to be the major irritation.

2. Cherenkov formula and the choice of radiator gas The well-known Cherenkov formula is 1 cos h" . bn For STP gas, n!1;1. Our choice of gas is perfluorobutane (C F ). It has a molecular weight 4 10 of 238, attractive chemistry and a large index of refraction (n!1"1.35]10~3). It is convenient and accurate to use the extreme relativistic approximation to the above Cherenkov formula and think in terms of the Lorentz c factor rather than b, 1 1 2(n!1)"h20" "h2# , c2t c2 where 2(n!1)"2.70]10~3, h0"h(for bP1)"52 mrad, c5"c threshold"19.2.

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The fact the c5 approximates cCM and equivalently, that h0 is approximately the laboratory mean particle production angle is a happy feature of C4F10. It is very well matched to our needs. As noted in Table 1 our average kaon momentum is approximately 15 GeV/c. More that 90% of the kaons that we will confront are between 4 and 60 GeV/c, i.e. within a range 3 or 4 times larger (or smaller) than the mean. There is also a strong correlation between production angle and momentum. The kaons produced between 10 and 50 mrad are “stiff” while kaons produced in the backward hemisphere are “soft.” Clearly the forward hadron density is markedly higher than the density at large angles. For the large angles, it is cost effective and non-compromising to use a larger cell size and tolerate a lower resolution for the measurement of the ring radius — it is a region of substantially lower ring overlap and undemanding p-K separation. The momentum thresholds for p, K and p(pM ) are 2.7, 9.6 and 18 GeV/c respectively. From approximately 3.5 GeV on up, p’s should disqualify themselves by producing recognizable rings. In the interval 4—12 GeV/c, kaons will be identified as being not pions. Above 12 GeV/c, kaons will positively identify themselves. We are relatively little concerned with a failure to effect a positive rejection of protons (or antiprotons). They are not plentiful enough to constitute a serious dilution problem. Over the bulk of the solid angle covered by the RICH, at the larger backward CM angles, few of the kaons are above threshold. In this expanse, the RICH is really operating as a highly selective and segmented threshold counter, tagging and rejecting pB, lB and eB.

3. Properties of the ring imaging focal surface 3.1. Location of the rings The paraxial approximation is generally excellent for the stiffest hadrons where the velocity resolution is most seriously challenged. It is convenient to visualize the charged tracks as “reflecting” from the successive mirrors and mapping onto the focal surfaces. These are the ring centers. Let (h ,h ) H V V. DETECTOR AND TRIGGER PROJECTS

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be the production angular coordinates of a particular track. The physical location of the ring center at the focal surface will be (h $[0.7 GeV/c]/p,h ), H V where p is the track momentum in GeV/c and the particle charge determines the sign of the deflection. Note that the ring axis location does not explicitly depend on the position of the magnet along the beam axis. The rings will be quite circular except for those at larger angles and with correspondingly lower momenta. Corrections for elliptical shape and ring blurring can be effected in offline analysis although as noted earlier, the velocity resolution demands for these tracks are generally less demanding 3.2. At what angles do the photons strike the focal surface? It is important to the design of the focal plane light collection to understand this geometrical consideration. The light is not normally incident. There are three sources for the non-normality. (i) The spherical mirror tilt produces a 2(150) mrad component to the vertical incident angle. It is a common effect and can be collectively adjusted out by a common rotational offset of the entire focal surface at the expense of introducing a degree of defocusing. Fortunately, the depth of focus of the focal surface is sufficiently accommodating and permits reasonable liberties to be taken with the focal surface. We shall expand upon this in Section 3.3 below. (ii) There is a horizontal contribution to the non-orthogonality angle from the magnetic deflection: (0.4) (2) (0.7 GeV/c)/p. The first component is the ratio: Magnet center location 4.5 m " "0.4. R 11.5 m It is of concern for low momenta hadrons. (iii) The Cherenkov emission angle itself contributes a component 42h . It contributes with aziC

muthal symmetry. It requires some explanation. We do not know the z location within the radiator of a particular photon emission. However, our radiator is not particularly thick. It occupies the last 25% of the distance between the target and the focal plane. Consequently this contribution to the nonorthogonality is confined to the polar angle interval 2(1.0!0.75)h . For b"1 the average is C 87 mrad. The locus of all possibilities may be described by visualizing the angular region subtended within two concentric cones constructed with their common axis along the direction to the surface determined by considerations (i) and (ii) above. The two boundary cones have half angles of 2h and C 1.5h . C A more detailed description of this photon incident angle consideration, documented with extensive Monte Carlo simulation, based on realistic production models, will be published elsewhere. Such studies have been carried out and continue to be refined. The bottom line is that our present focal surface design intercepts photons which are predominantly 100 mrad or less with respect to the normal. 3.3. How forgiving is the depth of field of the focal surface? Given unblemished tracking information, the RICH analysis begins with the complete prediction of the ring pattern for each assumed charged particle species. This statement is true except for one caveat: the always unknown z position of photon emission. But as noted above, the range of uncertainty is (0.25)2h which results in p +7 mrad (for C h a b"1 particle). A 20 cm focal plane offset results in a 1.4 mm uncertainty to be convoluted with other uncertainties (cellularization, spherical aberration, etc.). It is seen that $20 cm offsets do not markedly damage the velocity resolution. For this reason it is practical to flatten and rotate the focal surface. Each of our dual working photodetectors consists of five discrete, flat rectangular “supermodules.” The “five” dimension is the horizontal one. The outer two modules are rotated to better approximate a sphere. The focal surfaces are thus polygonized cylinders.

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4. The focal plane detector

4.1. The PMT framing problem

The original RICH photodetector design called for TMAE gas. After extensive design and test work, a decision was made in June of 1996 to switch to a design based on the exclusive use of PMTs. It had become apparent that the effective lifetime of the gas-based photodetector did not meet the design criteria. Simultaneously, a new generation of commercial multianode PMTs became available. The incremental cost increase proved to be less than initially feared due to the following. The original number of cells was 90 000. The new design called for 28 000. This economy was realized due to the efficiency of photodetection in a PMT based system being twice that of the TMAE based system. It became possible to greatly reduce the active area of the system (by excluding portions of the low density, large angle regions of the focal planes). Our original effective cathode area coverage was 6—7 m2. Our final scheme is approximately half as large — the total area is 3.25 m2 (see Table 3). With a final number of photoelectrons of 30—35/2p ring/b"1 particle, it had become clear that the relatively few large angle tracks with low momentum and with sparse overlap background could be successfully identified with a half or even as little as a third of the ring within the fiducial area of the detector. The reduction in the number of electronic channels, elimination of fused silica windows with awkward seals, elimination of a TMAE gas recirculation system, associated reduction of radiator gas purity requirements (TMAE is sensitive to photons of wavelength 180 nm while the PMTs are sensitive to photons in the range 280—600 nm) and other features produced savings in the cost which largely cancelled the cost increase due to the PMTs.

By framing problem, we mean the loss of efficiency that accrues as a consequence of the inevitable inability to have a large photo-sensitive area without dead gaps. Even very close-packed R5900 PMTs (on 30 mm centers) would be sensitive only over 35% of the occupied area on focal plane. The more temperate lattice choice of 36 mm which we have adopted allows for some thin magnetic shielding and electronic connections with other than custom designed connectors. Thus our photocathode covers 25% of the occupied focal plane area. The type of reflective light cones which are commonly employed with single anode PMTs is an inappropriate addition in our case. Our requirement was, and is, to take a 36 mm] 36 mm area of the focal plane, demagnify it by a factor of 2.05 in each linear dimension (a factor 4.2 in area) while maintaining a reasonable accurate mapping from a 36]36 mm2 area of the focal plane to the corresponding 18]18 mm2 area of the PMT cathode. For a few months we studied the possibility of using a set of small tapered UVT injection molded light guides. By Fall of 1996, this solution had evolved into a double lens system. It was simpler and more efficient than the light guide system — the loss due to reflection was larger but this was more than offset by decreased absorption and the elimination of losses along the edges of the light guides. The commercially molded lenses are of good optical quality without any hand polishing. The lens doublet serves either the M4 or M16 PMTs interchangeably. That we actually demagnify from 36 mm down to 17 mm (rather than 18 mm) is in deference to second order aberrations and edge losses which might result from slight misalignments. The submatrix of cells (4 or 16) are thus not exactly equal area cells — a very minor consideration. Jumping to the bottom line, the overall efficiency of the two lens telescope is 65% (15% of the loss is due to reflection, 15% to absorption and 5% to miscellaneous geometrical losses). The figure of merit for the lens system, confirmed by bench measurement, is

Table 3 PMT usage in HERA-B RICH Hamamatsu R5900 tube type

Number

Coverage (13 cm2/PMT)

Number of cells

M16 M4 Totals

1500 1000 2500

1.95 m2 1.30 m2 3.25 m2

24 000 4 000 28 000

Light gathered with lenses 65% " "2.6. Light gathered with no lenses 25%

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Fig. 2 shows a schematic of the focal plane assembly. A sturdy supermodule fabricated from high l steel, laser cut and bonded with black-casting epoxy provides the fundamental mechanical support. At the rear is an electrical assembly which anchors the PMTs and transmits signals from the ASD8 amplifier-shaper-discriminator unit to the digital pipeline units. At the front of the supermodule are the lens units. At the focal surface, there is positioned a square array of planoconvex ( f"95 mm) field lenses (35.3 mm]35.3 mm area). The remaining 0.7 mm] 142.6 mm area on the boundary is reserved for epoxy and Al partitions which support the lenses. At the focal plane of this lens is positioned a symmetric biconvex collector lens ( f"30 mm) of diameter 32 mm. The angle of incidence at the field lens is translated into a radial position at

the collector lens (parallel rays entering the field lens go to a particular distance from the center of the collector lens). Only rays parallel to the PMT axis will travel through the center of the collector lens. 45 mm downstream of the collector lens said rays will have diverged to occupy a 17 mm]17 mm area. But as noted earlier, there are a family of possible entrance angles with respect to the entrance symmetry axis. Without the collector lens, the image of the family of possibilities would be rather blurred. The location of the focus for off axis rays is directly proportional to their angle. The focal length of the collector lens is precisely the strength required to “kick” the rays sideways so as to produce a first order corrected picture. Both lenses are aspheric and correct to the 4th power of the radius. Space does not permit a leisurely description of the lens system and ray tracing studies which document their behavior. We pass from this abbreviated description to a documentation of the system performance, This is provided in Fig. 3a. A 12]12 array of simulated photocathodes has been photographed from the radiator gas side of the field lens with the camera lens 3.0 m from the plane of field lenses. In this time reversed world, the photocathodes appear properly magnified (by 2.05) over a range of angular departure from the normal. The recording camera is deliberately placed only 1.5 m from the lens plane in the second photograph (Fig. 3b). The system is seen to cut off at approximately 140 mrad — rays are beginning to miss the 32 mm aperture of the collector lens.

5. Photocathode efficiency and resolution The number of found photons, N, is given by the following formula:

Fig. 2. Layout of the focal plane photon detector. In the center can be seen the supermodule which constitutes the principal structural support as well as magnetic shielding for the multianode PMTs. In the foreground, UV transmitting acrylic lenses are positioned. In the rear are fastened the voltage connections, front-end electronics and signal outputs. More detailed description can be found in the text.

e N"N h2¸e e e 0 0 M1 M2 L%/4 R%&-%#5*0/ M*4#. 1 2 "(100 p.e./cm) (280 cm)(0.9)(0.9)(0.85)(0.8) 192

A B

+35 where e ,e are the mirror reflection efficiencies, M1 M2 1!e is the lens doublet reflection L%/4 R%&-%#5*0/

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Fig. 3. Demonstration of the focal plane multianode PMT light collection system. For demonstration purposes, a simulated array of Hamamatsu M16 PMTs with 18]18 mm2 photocathodes is backilluminated. The cellular structure is viewed from “radiator” space and recorded on film. A square array of (12)2 lens telescopes is positioned in front of the PMT array. An extra two bands unequipped with lenses are visible at the top. (a) has the camera lens located at 3 m from the (43.2 cm)2 array. The desired cellularization is demonstrated. Note that a pattern of letters reading: HERA-B, DESY, NU has been superimposed on the mock cathodes. Each letter is rotated by 180". (b) was taken at a distance of 1.5 m. It is demonstrated that the corners fail to be recorded. In other words, the lens system is effective up to approximately 140 mrad. At larger angles the collector lens aperture is blocked. Virtually, all of our photons strike the focal surface at angles substantially below this cutoff.

losses. N is the number of detected p.e. per cm 0 which is given by

P

N "300 e dE+(300)(0.2)(1.5 eV)+100. 0 C!5)0$% (This integral includes UV absorption losses in the acrylic windows and lenses.) Fig. 4 illustrates the absorption curve (8% reflection losses are included as is industrial practice) for the UVT acrylic material used for the vessel windows. If is fairly well matched to the PMT response. The average path distance through the acrylic lens system is 11—12 mm. The absorption in the cast material is slightly worse than that in the sheet material of nominally the same material composition.

5.1. Resolution From the fundamental formula c~2"(h !h)(h #h), 0 0 it is easily seen that p(ring radius) p 1 " h" (c /c)2. (ring radius) h 2 5 0 Now p "(cell size)/J12"2.6 mm (for '%0.%53: 9 mm]9 mm cells). Hence, p /h "2.6 mm/ h 0 30 cm"0.9%. The uncertainty due to chromatic aberration is approximately 0.5%. But we have multiple p.e. per ring (N ) and we gain as 1.%. 1/JN . 1.%. V. DETECTOR AND TRIGGER PROJECTS

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Fig. 4. Absorption curve for 6.35 mm UVT Acrolyte Sheet.

Fig. 6. Vapor pressure versus temperature for C F . 4 10

extinguish fires. A 2 m3 refitted propane tank is quite suitable for auxiliary storage. The gas system, operating as it does in the liquid phase, is rather compact utilizing relatively small bore plumbing. Only upon entering and exiting the detector vessel’s large diameter tubing is the gas permitted to vaporize. The gas system is being assembled at CERN under the supervision of Michel Bosteels and his group. Fig. 5. Several measurements of the refractive index of C F . 4 10

7. Conclusion If the percentage ring resolution is 1/2% or better then (c /c)51/10 seems achievable. A kaon with 5 c"200 is a 100 GeV kaon. The present limitation on particle ID will be dictated by background hits due to overlapping rings. Regions of the focal surface with occupancies exceeding 40% are functionally bleached out.

6. Radiator gas Figs. 5 and 6 display the vapor pressure curve and the refractive index of the C F radiator gas. 4 10 It is seen that the gas is readily liquefiable at a few atmospheres of pressure. It is a safe gas used to

In this brief overview of the essential characteristics of the HERA-B RICH system there are important topics which can only be noted. We have glossed over voltage distribution to both the PMTs and the electronics, the architecture of the signal processing (including both the front-end electronics and the offline software), the PMT performance, diagnostic systems (light flashers, index of refraction monitor and the gas system) and years of test beam studies. Our schedule calls for the commissioning of the RICH system toward the end of 1998. The author’s participation in HERA-B is underwritten by D.O.E. Grant DE-FG02-91ER40684.