Subject: forthcoming test exposure in CHORUS
Dear Collegues,
This is to summarize the motivations of the forthcoming "Lead-emulsion
target" tests at the CHORUS target position,
and to inform you in some detail about the design of the target structure,
and the tentative programme for pouring, stacking, installation, data taking,
dismount and processing.
Aims and boundary conditions
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The CHORUS apparatus is taking data for the last time this year, and it is not clear if, and how long, the existing WANF will provide any further neutrino beam sometimes after 1998. Next-round experiments to detect nu-tau appearance with some hybrid apparatus could be performed in the forthcoming years, however, on some baseline and, likely, with larger target mass.
To produce, handle, process and scan the large amount of emulsion afforded by CHORUS, adhequate financial resources, manpower and experimental facilities (pouring, processing, and scanning labs) had to be provided. It is not trivial to scale-up further. A mixed target design could be a viable option for short-baseline experiments, and it is compulsory for medium-long baseline. On the other hand, in order to provide evidence for a hypothetical, rare and elusive process as the interaction of a nu-tau with subsequent production and decay of a tau lepton, much care must be exercised in the design of the target. The conceptual structure of the "ECC" was presented sometime ago by professor Niwa, and CHORUS members envisaged a possible test exposure. Later on, in the framework of TOSCA, the proposal arised to build a prototype module based on silicon detectors and a mixed (W or Pb/emulsion) target , to be exposed inside the NOMAD magnet in 1999.
However, such a test turned out to be not feasible (leaving aside other
difficulties, no neutrino beam in 1999). So, TOSCA members took into consideration
the idea of testing at least a mixed target design in CHORUS, a great advantage
being that the procedure to interconnect fiber trackers to the emulsion
is rather well understood in the CHORUS framework. The CHORUS Collaboration
kindly accepted to host such a test. Finally, it was proposed in may at
the CHORUS meeting to add in a fully parasitic way another target module,
somehow inspired by the early ECC concept, one step further away from the
pure and massive emulsion target. The two target designs, namely a Lead
and bulk emulsion array and a Lead and thin double-coated emulsion array,
are described below.
The Pb/thick emulsion array
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A key concept inspired the mixed-target design: interactions inside a thin metal sheet in close contact with emulsion plates can be reliably RECONSTRUCTED by tracking back, whereas a weak topology such as a 'kink' in connection with a rare signal such as the appearance of a nu-tau, must be not only reconstructed but also SEEN. Indeed, close inspection of the decay point is mandatory in order to rule out scattering background, e. g. for hadronic decay channels. On more fundamental grounds, also for a clean channel as the decay of a tau into a muon, relatively small kinks could be the artifact of misalignement, for a large-scale apparatus and with a long-range 'blind' extrapolation.
For the sake of a practical implementation, the key quality factor being the ratio of VISIBLE over TOTAL potential decay path, a set of materials were considered under the assumption of getting roughly half of the neutrino interactions inside a thin, dense, and not very expensive metal sheet, and half inside a 'standard' bulk emulsion plate (i.e. 350 + 350 micron on the two sides of a 90 micron thin plastic base). In conclusion, the choice is restricted to Fe, Pb and W.
The relevant parameters for the target composition are listed below.
| Emulsion | Iron | Lead | Tungsten | |
| Density (g/cm3) | 3.7 | 7.9 | 11.4 | 19.3 |
| Standard unit (micron) | 350+350 | 328 | 227 | 134 |
| Interaction length (cm) | 36 | 17 | 17 | 9.6 |
| Radiation length (cm) | 2.9 | 1.8 | 0.56 | 0.35 |
| Radiation length for one standard unit (cm) | 0.024 | 0.018 | 0.041 | 0.038 |
In short, a standard unit will provide the same number of neutrino interactions as one 'bulk' emulsion plate. The thinner (denser) the unit, the higher the ratio visible/total path for a mixed target (e.g. several lead+emulsion pairs) of the same effective mass of a pure emulsion target. On the other hand, by increasing the atomic number of the passive target a higher fraction of a radiation length will correspond to a standard unit.
For the test inside CHORUS, the mechanical constraint is to fit into the volume of a standard target module, i. e. about 36x72 cm2 cross section and about 3 cm depth along the beam. After a search on the market, the order was placed for Pb rolls 250 micron thick, as the closest available approximation, with rather low cost, to the reference thickness of a standard unit.
In order to interface to the CHORUS target tracker predictions, as usual,
a 'changeable' sheet should be put downstream of the target, and a 'special'
sheet should be attached to it. For this test exposure, moreover, it is
proposed to add:
The thickness of the base for special and changeable sheets used to be 800 micron in CHORUS. It would be nice to explore the functionality (from the mechanichal, optical and scanning point of view) of a somewhat thinner base, of about 500 micron. Precoated bases of the proper size will be looked for.
In conclusion, given the costraints, the proposed target structure is
the following (upstream to downstream):
The overall mass is of the order of 38 Kg, out of wich about 19 of Pb, to be compared to about 25 Kg of the standard CHORUS target. Thus, about 50% more interactions will be obtained by filling up the same volume, half of them due to Pb nuclei. The visible path (i.e. emulsion/total thickness) will be 67% in the target section, to be compared with 89% (due to the blind path inside the plastic base) for the pure emulsion target. The relative loss of visibility is of the order of 24%, but it refers to short-decay path for neutrino interactions inside Pb. As far as showering is concerned, the target will be about 1.8 Xo thick (1.2 due to Pb), to be compared to about 1 Xo of CHORUS.
The Pb/thin double coated emulsion array
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With the aim of enhancing the study of interactions in Pb and decoupling
each target from the following, the above design could be expanded in the
longitudinal direction by inserting some light spacers between thin double
coated emulsion plates. The visible path will be much more reduced, and
some redundancy of tracking will be needed to
be able to detect (and 'defend') kink decay topologies. As a possible
design (by no means unique nor optimized) I proposed to set-up a target
of the standard CHORUS overall thickness (3 cm) and size (36x72 cm2), structured
as:
- 10 modules of:
Planning
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I) Passive target. The Pb target needs to be: unrolled; made flat; cut to the proper dimensions; cleaned ; surface passivated (nickel bath) in order to allow close contact with emulsion. A major inconvenience will be the size of the roll, 30 cm, mismatching our standard 36 cm. Some "bricolage" will be needed to arrange a passive layer (e. g. one sheet of 30x72 cm2 and one strip of 6x72 cm2 put aside).
II) Pouring. We should produce 26 thick plates 36x72 cm2. Thus, 13 large plates (72x72 cm2) should be poured (first side, drying, second side, drying, cutting). The pouring operation should last 2 or 3 days per each side. About thin plates, we would need 35 of them, on different bases (300 micron, 500 micron, 800 micron).
The precoated plastic bases need sometimes cleaning before use.
Pouring is made on two adjacent plates per pouring table. The preparation
cycle is similar to that needed in the past for a full set of changeable
sheets.
One point on the critical path is reassembly of the cutting machine, that was dismounted after the CHORUS exposures. The status of the cutting edge and the mechanical reference bars should be carefully checked well before the beginning of pouring.
The target design relies on the availability of 500 micron thick precoated base, 36x72 cm2, some 15 sheets needed, and of 300 micron thick precoated base, 36x72 cm2 or bits of 36x36 cm2 to be recombined inside the target module. Some 800 micron base is left, and can be used for changeable sheets, and also for the thick emulsion stack if the 500 micron is missing. The thin target design cannot be implemented with very thick base sheets.
In order to produce 26 standard thick plates 52 Kg of 'concentrated'
Fuji gel are needed. For the 35 thin double-coated
plates, some 20 Kg of gel are needed. New gel was already delivered
to CERN middle of may (9 boxes or 45 Kg). We should carefully check the
exact amount of gel left last year. Probably we are slightly short of gel,
and one or two more boxes could be needed.
III) Mixed target assembly. The Pb sheets will be finally cleaned and assembled under vacuum package with emulsion plates. As far as spacers for the second stack, some suitable thin light sheets are being looked for. Another job to be done before installation will be to set up the usual changeable sheet mosaic, with the two positions corresponding to the targets equipped and the other 6 out of 8 covered by dummy sheets. At that moment, the final positions of the two targets should be defined, the obvious choice being module 4 (most downstream) and two central positions.
IV) Installation at the experiment inside the target holder. This operation, implying air bags and displacement of the target modules, was mastered in the past by W. Flegel and coworkers. They should be prevented in due time, and a suitable date for installation should be stated accordingly.
V) Data taking. We aim to collect some 2* 103 CC interactions in the thick target module, by 3-4 weeks exposure at high intensity. The vital part of data taking will be the neutrino interaction trigger and the read-out of the TT fibers. The muon spectrometer tag would be highly desirable for the interpretation of the located events.
The calorimeter would add precious information for the pre-showering effect of the 1,8 Xo thick target. Some interference could come from the extra target put in place of the DT, so that, if compatible with the aim of the planned measurements with several targets, it could be better to put the mixed thick target in front of a light one preferentially. Anyway, this can be agreed just before the installation.
For simplicity, one could install early in august, and leave the material in till the end of the neutrino run. The scheduled use of the X7 test beam line should be checked, just to avoid the unpleasant surprise of some peak usage with high energy negative pions.
VI) Dismount and processing. This operation could take place around mid september. Processing of thin plates is simpler, similar to a changeable sheet cycle. The processing itself could take 3 days for the 35 plates. For thick plates, the processing procedure, including preparation and all the drying steps, will last about 10 days. Up to 16 plates can be processed each time, and 2 cycles of processing will exhaust our target, out-of-phase by one day each other.
The manpower needed to take care of the preparation is in the range of 5-6 people for 10-15 days. For processing, 3-4 people for 10-12 days. At least two experienced people must be always present (Phycisists and Technicians).