Test of an Internal Wire Target at the Hera Proton Ring

TEST OF AN INTERNAL WIRE TARGET AT THE HERA PROTON RING

THOMAS LOHSE MPIftir Kemphysik ffeiddberg, 69Of?9, ccrmcng

1. INTRODUCTION Shortly after the HERA machine started its luminosity operation a working group was formed to study the possibility of exploiting the HERA proton beam for a high statistics B experiment able to discover CP violation in the B system. The efTort resulted in two detailed reports I,’ submitted to the DESY PRC. HERA has four intersction paints, two of which are occupied by the two large experiments HI and ZEUS, and one which is planned to be used by the HERMES experiment. The fourth interaction zone is currently available and suited to house a dedicated B de$ector. In principle, HERA has two independent rings and one could investigate the possibility of proton proton collisions (with asymmetric energies). Since this would, however, interfere with the routine e-p operation, we concentrated on the fixed target option in which protons from the beam halo intersct with massive internal targets, e.g., thin wires grouped around the beam center at distances of about 4 r.m.s. beam sizes. In the fixed target mode, the center of mass energy is only slightly above 40 GeV. This is quite close to the b&threshold and thus the B cross-section is tiny; about 10 to 20 nb are expected theoretically. The charm cross section- a dangerous background- is three orders of magnitude larger and the total inelastic cross section dominstes by 6 orders of magnitude. In order to get a precision of 0.05 in sin(2p), one thus has. to produce a huge number, lo”, of inelastic events, requiring 5 snowmass-years of data taking at interaction rates of about 39 MHz. There rates are in principle achievable at the HERA proton ring: With a nominal 2. 1013 protons stared and lifetimes around 50 to 100 hours, natural beam losses correspond to 50-100 MHz. We have to require, however, that the internal target is very efficient and able to absorb around 50% of all halo protons (which are about to leave the machine anyhow). Since the feasibility of an internal target is of primary importance, beam tests were set up at the HERA proton ring. The mein results of the 1992 tests are summuizcd in the following.

a. EXPERIMENTAL SET-UP The choice for the target location, llgm upstream of the center of the HERA w-t hall, was dictated by practical necessities like finding II beam pipe section void of other

641 machine elements. Relevant parameters for optics, beam and target are summarized in PI Table 1. The optics are quite different from that for .the proposed final target srrsngcment; n cl most significantly the relatively large horizontal 0-f unction of 93 m exceeds the maximal value I, \I” of 50m assumed for e B experiment. In addition, other (position independent) parsmeters, the total current in the machine, the machine aperture, and the number of target wires, were smaller than in the design in reference’.

Table 1: Parametera of the target wire and the proton beam at the ted location 1lSm 1ow nr upstream of the HERA west hall. II IDP L Proton Beep: Figure 1: Sketch of the expedmental set-up. Sl, S2: scintillatars; Sh: shower counters; Beta function p. =96 m Vl, V2: scintillators used as VETO counters; T: target. Alpha a* = -1.68 Emittsnce tr = 9. 10-O red m t, = 9. lo-’ red m Spatial dispersion d. = 0.82 m d. = 0 bunches. Data on the background detected by Si-pin-diodes in the vicinity of the collima- Angular dispersion d., = 24 mrad d.. = 0 tors and from ZEUS (scintillators C.5, proton-gated) and Hl (veto weI1) were continuously Beam size (r.m.s) cz = 0.9 mm or = 0.5 mm displayed in the control room and were partly available off-line. Number of bunches 10 (with 96 ns spacing) In total we performed 7 experiments with the wire close to the beam, corresponding Typical current 1.6 “A (= 2. 10” protona) to L IotaI of 22 hours of data taking. In particular, the wire was several timea systematically Target Wire: moved towards the beam in small steps, while HERA was operated in normal luminosity Material Copper mode. In these situations, where the test experiment was running in a purely par&tic Diameter 100 pm mode, the main collimators were positioned at about 5 1.m.8. beam widths from the nominal proton beam position end were not moved. In two occasions during the power saving hours Interaction length 151 mm in November, when only protons had been stored in the machine, the collimators could be Radiation length 14.3 mm opened with the wire already positioned in the beam halo. The beam lifetimes were~typically around 100 h when the target ,u retracted. With- The target was mounted verticeIIy on e movable fork. It consisted of a 1OOpm thick out e-p collisions, the lifetime wea probably much larger, but no messurement beyond 100 h copper wire, followed by a second spare wire and finally a 100pm thick copper foil. Since was available. The presence of the target reduced the lifetime to typically 30-SOh, either the first wire was not damaged during the tests, the back-up targets were never used. The with or without e-p collisiona, and no distinct differences between these two aitustions were fork war driven by a stepping motor (normally used for collimators) with a step size of 3pm. observed. It was directly controlled via the collimator control panel from the HERA control room. The target area we.8 very close to the main proton collimators. These beam scrapers were The data presented here were taken from those running periods where the conditions situated about 6 m downstream of the target. Secondary collimators existed 214 m and 259 m were rather stable, i.e., neither jumps in the beam position nor violent besm losses in either downstream of the main collimators. the proton or the electron beam occured. Fig. 1 shows the whole experimental set-up. Downstream of the target (T) and e beam pipe section of increased diameter (ZOOmm), telescopes were placed above, below s. INTERACTION RATES IN WIRE SCANS and at both sidea of the beam pipe. Each of the four telescopes consisted of two plastic scintillators, a smaller one (Sl, 40 x 40mm’) positioned at 1.4” from the target, end e. In the parasitic experiments, the wire wan carefully moved towards the beam in emaIl larger one (SZ, 80 Y 80mm’) at 1.8 m, followed by II acintillator-lead shower counter (Sh) of steps of 30pm. An example is shown in Fig. 2e, which displays the horizontal wire position the same active ares end a depth of 18 radiation lengths. After the first measurement, the ea c function of time. At time 2OOOee first increase of the trigger rate (Fig. Zb) wee observed. upper telescope wea dismantled and its two scintillators were plsced 0.3m upstresm of the Simultaneously the percentage of triggers with the VETO counters fired (Fig. 2c) dropped target M “VETO” counters (Vl, VZ). Th 1s rearrangement sllowed us to determine to what from 90% to 35%, demanstrsting that iI& incrcaae in the trigger rate wea not due to beam extent the telescopes were triggered by beam related beckground and not by intersctions in beckground (which could have possibly been created by the disturbance by the wire). The the target. obvious interpretation ie that at ibis time the wire just moved out of the shadow of the During the whole running period HERA WM filled by a short train of 10 consecutive collimator and started to scrape away protons from the beam hdo. bunches (with e hooch spacing of vx 96ns). The arrival times st the test experiment were In the following hours the wire was moved another 14 timea towards the beam, cu GUI derived from e close-by besm pick-up. Only the bunch-crossing (BX) signal of the Iead- be seen in Fig. 2. After each step, the coincidence rate first increased sharply, accompanied ing bunch wes allowed to trigger the tclcscopea. This rulea out any pile-up from preceding by e corresponding drop in beam lifetime. Within minutes, the rate then gradually settled

642 5.4 c) Yeto rate 17.1 5.2 1 ij ‘3-l 1 5 *.a a) wire pas. Imml z/u +.5 zoo0 4000 6000 8000 10000 0 60 80 60 80 c b\ coincidence rote fkHZI , I TfJC channel TDC channel E°Cfl Figure 3: Time spectra of events, where one (hatched histograms) or both (open histograms) VETO counters had fired. One TDC channel corresponds to 0.6 ns. The spectra are shown for the wire retracted or fully moved in. 0I’ 2000 A,000 6000 BhoO 10000 0 zoo0 moo 6000 BOO0 10000 time Is1 time Is1 background events set both VETO taunters at once in almost all CBS~S, When the target Figure 2: Wire scan information venzu~ time: a) Ho&x& wire position; b) ORed coinci- is moved in, however, x 80% of the events set only one counter, showing the presence of dence rate using three scintillator telescopes; c) percentage of triggers with VETO counters a new low multiplicity component in the counters. in addition, in these events where only fired; d) background rate measured by the proton gated ZEUS s&tiUator C5. one VETO counter is set, the VETO signal is delayed by zz 3 ns with respect to the beam background. This could be due to slow particles coming from the target; it takes the beam 1 ns to arrive at the wire target, and secondary particles near the velocity of light would to a new equilibrium and the beam lifetime recovered’. The equilibrium rate increased reach the VETO counters Ins after the interaction of a halo proton in the wire, i.e., after a steadily while the wire was moved in, showing its increasing efficiency in scraping protons total delay of 2 ns. with increasing distance from the collimator shadow. The percentage of triggera with VETO In a more quantitative analysis’ of the rates of single and double VETO hita we counters firing dropped to 5% after the first few wire moves and stayed at this level for the could extract the fraction of background events triggering the detector. The measurements rest of the experiment. indicate that the wire does not increase the background but cleans up the beam (at least The transient after a wire step can be semi-quantitatively understood. The wire locally), such that the background rate is reduced by a factor 2 to 3. cleans up the halo with bet&on-amplitudes beyond the wire position and the beam profile The interpretation of the VETO signals requires the existence of slow particles moving i. slowly re-adjusting to the new boundary condition given by the wire. The shape of the rate into the backwards hemisphere. The origin of these particles is not obvious, since a proton dependence aftcr,a wire step is identical to that observed on the pin-diodes after a collimator interaction with a nucleus in the wire produces secondary particles strongly boosted in the is moved in. The only quantitative difference ia the transient time which is about a factor forward direction. Simulations based on the FRITIOF3 generator and a GEANT detector of 5 longer for the wire than for the collimator. This ia no surprise since the wire is acting simulation in fact predict that leaa than 1% of the interactions should produce B particle like a semi-transparent scraper and particles with amplitudes beyond the wire position thus hitting a VETO counter. have a finite lifetime. One possible explanation is the production of nuclear fragments, not included in the During the whole operation the data taking of the other HERA experiments was not disturbed. Fig. 2d shows the ZEUS background rate, which stayed constant throughout the event generator. These fragments are protons with kinetic energies below a few hundred MeV, observed by several experiments in hadronic interactions over a wide range of center run and WQ.Sonly little affected by the wire movements. of mass energies’. At the smallest kinetic energies (below 25 MeV) these fragments are produced isotropically. With increasing energies they acquire a slight forward boost. The 1. EVENT TOPOLOGY multiplicity produced in interactions with nuclear targets is a strong function of the mus It was shown in the last section that the rate of VETO events drops quickly to about number of the target nucleus. Heavy target materials, like the copper wire used in our teat 5% of the total trigger rate when the wire moves in and then stays at this level independently experiment, therefoze have a certain disadvantage as compared to lighter targets, since the of the total interaction rate. There are two possible origins of this 5% remnant: slow moving fragments can produce considerable radiation damage to detector elements. An independent hint for the existence of extra particles not properly described by l The wire disturbs the beam and thus produces beam background events, the rate of the FRITIOF generator is obtained from the analysis of the event topology in the forward which scales exactly with the interaction rate. hemisphere. It is observed that the trigger telescopes are more strongly correlated, i.e. fire more often simultaneously, than expected from Monte Carlo events. The events are thus l In the real inelastic interactiona in the wire, a significant number of tracks are emitted backwards, so that a VETO counter ia hit in roughly 5% of the cases. more crowded than expected. This effect is not yet quantitatively understood.

The following analysis shows that the second effect is most probably the dominating one. 6. TARGET EFFICIENCY Fig. 3 ahows the time spectra of VETO hit. in events where one (hatched histograms) or both (open histograms) VETO counters fired. When the wire is totally retracted, the The interaction rates measured in the test run (10 to 100kHz) are small (yi compared to the requirements for a B experiment (10 to 50MHz). Th e main reason is the fact that ‘We rheckcd in s dedicated crperimcnt, in which the wire stayed in one position for &ou* 30 ndnutc~, the machine is still run+ng with a small fraction of the final design current. A quantity ti3.t the r.tc redly reach” a” equilibri”m.

643 6. SUMMARY The first phase of test experiments using internal wire targets has led to a proof of principle of the technology, together with rather detailed understanding of the mechanisms relevant for the interaction of wire targets and beam halo. The main results are: . Interactions of halo protons with the wire have been observed with rates up to 100 kHz.

l The wire does not produce large beam background; it reduces the beam lifetime to typically 40 to 50 h.

l Tracking simulations are able to quantitatively predict the observed interaction rates. Extrapolating to design parameters, the models predict rates sufficient for a major E 4.7 4.8 4.9 5 5.1 5.2 5.3 experiment at HERA. torp.1po*lion tmm, Figure 4: Target efliciency obtained from the observed coincidence rate as function of the l Transients of rates observed directly after a wire movement resemble those expected for target position (relative to an arbitrary zero-point near the aswxned center of the beam * semi-transparent scr8.p.x. pipe). Also shown is the efficiency predicted by tracking simulations. Nonetheless, a number of problems remain. The most serious ones are the unexpected event topology, suggesting that the events are more crowded than expected, and the time which is more useful than the absolute interaction rate ia therefore the target efficiency, i.e., structure of VETO signals, suggesting the existence of a component of more or less isotrop- the fraction of protons interacting in the wire as compared to the total number of protons ically produced slow particles. These problems will be attacked by B more sophisticated leaving the machine. Target efficiencies of 5OYo are sufficient to reach the desired interaction target and detector, which have been installed for the 1993 run. We are confident that these rate for design proton currents and design lifetimes of the proton beam’. tests will give the final proof of the feasibility of a halo target for the planned I3 experiment. The target efficiency, CT, can be computed using the number of protons, Npy stored in the machine, the lifetime 7p of the proton beam, and the measured interaction rate on the 7. REFERENCES wire, IP,*: 1. H. Albrecbt et al., An Ezpwimenl to Study CP Violation in the B System Using an cT = _ 1 .3 R”“” Internal Target at the HERA Proton Ring, Letter of Intent, DESY-PRC Q2/04 (1992). r., Np 2. A. Albrecht et al., An Ezperimcnl to Study CP Violation in the B System Using an In- Here, 6.re is the acceptance of the trigger telescopes, which has to be estimated by ternal Tovet at the HERA Pmfon Ring, Progress Report, DESY-PRC OS/O4 (1993). Monte Carlo simulation. For the test set-up we find c- = 43% based on the FRITIOF 3. B. Anderson, G. Gustafson and Hong Pi, LU-TP-82-20 (1992). generator and the GEANT detector simulation. 4. A. Abdushamilov et al., 2. Phya. C40 (1966) I, 2. Phys. C40 (1986) 223; The coincidence count rates of Fig. 2, always taken ten minutes after the last wire 3.A. Gaidos et al., Phya. Rem Lcll. 42 (1979) 82; movement, were converted to target efficiencies, shown in Fig. 4 as function of the target J.E. Finn et al., Phys. Rev. Lcll. 48 (1962) 1321; position (relative to the assumed center of the beam pipe). At about 5.24mm the wire leaves A.S. Hirsch et al., Phya. Rev. C20 (1964) 506. the shadow of the collimators and the target efficiency stsrts to become non-zero. At 4.7mm (ix., about half a r.m.s. beam width further in) the efficiency reaches 6 to 7%. A single particle trackins simulation’ was used to predict the target efficiency for the single wire target of the teat experiment. The prediction is indicated in Fig. 4. It is in good agreement with the measurements. WC used estimates for the actual machine parameters, in particular a tight collimator setting at 50. We also varied assumptions on the drift speed of halo particles aa well M assumptions on coupling and absolute beam position. The predicted target efficiency was found to be rather insensitive to these parameters. The target efficiency in the test experiment was limited by the large p-function and the tight aperture. In a control experiment with only protons in the machine (and cxpcr- imcnts switched off) we opened the collimators and reached interaction ratea of 100 kHz, corresponding to 15% target efficiency. The aperture was in this case still limited by an unidentified obstacle in the proton ring around 5.50 (horizontally).

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