Report on the Progress for The

R-99-05.1

Report on the Progress for the

m+®e++g Experiment at PSI

Collaboration for the m+®e++g Experiment at PSI

Nov. 2001

Abstract

This report describes the progress made in the preparation for the m+®e++g experiment.

1.  pE5 Beam line studies

In order to achieve the design goals of the detectors involved in the experiment [Ref. 1Ref. 1Ref. 1Ref. 1], it is also necessary to have a well understood beam transport system delivering a high intensity surface muon beam, capable of stopping in a thin target with a minimum of contaminant particles entering the detectors.

Since the pE5 beamline is the most intense source of surface muons, the choice was clear. However, there are two separate branches of the beamline, a "U"-branch feeding the pE52-area and a "Z"-branch feeding the pE51-area c.f. Fig. 1Fig. 1Fig. 1Fig. 1. Previous measurements on the "Z"-branch [Ref. 3Ref. 2Ref. 3Ref. 3], showed that a suitable number of muons could be transported to a final focus with the help of a large solenoidal magnet (PMC-magnet), after having passed a degrader, to separate the muons from the contaminant beam positrons.

Fig. 1 pE5 Area showing the two branches, "U" leading to the pE52-zone and "Z" leading to the pE51-zone, where the previous measurements were made. Also shown is the spectrometer, used as a cleaning section, where the present data were obtained.

The present results are based on a comparative study on the "U"-branch with the aim of measuring the phase space parameters, in order that an optimal transport system to the experiment could be designed. Another goal was to try to improve the separation quality of surface muons over beam positrons, by using a spectrometer, consisting of a bending magnet and two sets of quadrupole doublets, as a separate cleaning stage. Unlike the previous measurements, in which the degrading was done in a magnetic field, here, it was achieved in a field-free region, which apart from being optically superior, also allowed a bending magnet of higher bending power to be used.

A beam test, during the months of September and October 2001, was prematurely ended by a technical problem in the primary beam-blocker system of the pE5 beamline, resulting in the fact that the channel remains unusable until a repair is undertaken during the next accelerator shutdown. Nevertheless, the main conclusions deduced from the data taken could be reached and the provisional results are presented below.

1.1.  Test beam layout

The test beam layout is shown schematically in Fig. 3Fig. 2Fig. 3Fig. 3. Optically the degrader system should be located at a "waist" or focus, where the beam divergences are large enough to allow the contribution from multiple scattering from the degrader foil to be appropriately small. This dictates that the location has to be in the secondary beam-blocker of "U"-branch, just upstream of the quadrupole QSE41.

Fig. 323 Area layout, schematic and photograph, showing the spectrometer, consisting of the quadrupole doublets QSE41/42 and QSE43/44 as well as the dipole magnet ASL52, which was mounted on a moveable assembly running on rails. The degrader system is located upstream of QSE41 and is not shown in the figure. A 4-micron thick differential pressure window, made of Mylar was placed just after QSE42 in order to suppress rest-gas activation in the spectrometer. Also shown are the detector systems used, a small "pill" scintillation counter, mounted on an X-Y scanner. A scintillation counter telescope S1, S2, together with a large NaI(Tl)-counter. A set of fast multi-wire proportional chambers (MWPC1/2), used for tracking, and a profile MWPC.

Since the degrader had to be inserted into a movable safety element, which had to be rotated in and out of the beam, special precautions had to be taken, necessitating the construction of a cylindrical assembly which then had to be inserted into the beam-blocker by means of a two and a half metre long rod system from the outside and locked into position. To gain access to this location and to enable the changing of the foil/collimator the ASL52 bending magnet had to be removed each time, which was achieved by mounting the magnet on an assembly which ran on rails. This operation could only be performed when the accelerator was turned off.

Measurements were taken at two locations in the area, namely, at the entrance, immediately downstream of the QSE42 quadrupole magnet and at the exit window of the spectrometer, some 50 cm after QSE44. In total, thirty-one measurements were taken under different conditions (counter type, beamline tune, momentum byte, collimator size and degrader in/out). For the degrader measurements, a foil of 450 microns of Mylar was used to separate the initial 28 MeV/c surface muons from beam positrons of the same momentum. While the positrons were virtually unaffected by the degrader, the muons reached a central momentum of close to 23 MeV/c, with still sufficient residual range to exit the final vacuum window without stopping, see Fig. 5Fig. 3Fig. 5Fig. 5. For all measurements a 2 mm thick, CH2-collimator with either a 50 mm or 80 mm diameter hole was located at the same point as the degrader foil. This was used to force a higher vertical divergence of the muons at the degrader during beam tuning. Unfortunately, planned measurements without this collimator could not be carried out in the end due to the technical problems with the beamline.

Fig. 535 Digital oscilloscope output from a 1mm diameter "pill" scintillator, wrapped with only 25 microns of Aluminium and positioned after the spectrometer, showing a positron line (lower red distribution) and a muon line (upper red distribution). The intensity is proportional to the colour, red being the highest intensity.

1.2.  Measurement Techniques

Three counter systems were employed to measure the muon and electron beam intensities and profiles, as well as to determine the beam divergences. All counter signals, both time and pulse-height, were read-out event-by-event in addition to the radio-frequency (R.F.) signal of the accelerator, thus allowing both pulse-height and timing cuts to distinguish between muons and Michel and beam positrons.

Firstly, various sized "pill" scintillators, mounted on a Hammatsu metal-channel, miniature, photomultiplier(PMT) and wrapped with only 25 microns of aluminium foil, were used. These could be mounted on a remotely controllable X-Y scanner table. An example of the direct output signal from a 2 mm thick, 1 mm diameter pill scintillator is shown in Fig. 5Fig. 3Fig. 5Fig. 5. This demonstrates clearly that pulse-height discrimination can be used to distinguish between positrons and muons. To distinguish between Michel positrons and beam positrons the timing information from the counter, together with the R.F.-timing signal from the accelerator must be employed. Michel positrons, originating from the decay of the muon, m+®e+n`n, are not beam correlated and thus have a flat time spectrum with respect to the accelerator R.F., while the beam positrons that are generated in the production target are correlated and produce a peak in the time spectrum.

Secondly, a 1mm thick CH2-plate was used as a stopping target for the muons, which then decay and produced Michel positrons. These were selected by the trigger telescope counters S1,S2 and their tracks reconstructed back to the target by the chambers MWPC1/2 while their energy was measured in a large, single-crystal NaI(Tl) counter, see Fig. 7Fig. 4Fig. 7Fig. 7.

Finally, a profile MWPC system was used to also measure the positron beam profile and divergences.

Fig. 747 Pulse-height spectrum obtained from the NaI(Tl) counter, after correction for dead-time and track reconstruction in the MWPCs, superimposed is the Michel spectrum smeared by a 6% energy resolution at 52 MeV.

1.3.  Results - Intensities and Phase Space

All quoted provisional results[1], unless otherwise stated, are normalized to 1800 mA of beam current on a 6 cm long Target E and a momentum byte of 6.5 % FWHM. Measurements showed that without the degrader and a 50 mm diameter CH2-collimator, a total muon rate of:

Nm+ ~ 3.5 · 108 m+ s-1

could be transported up to the spectrometer, whereas at the same location the beam had a total positron rate of:

Ne+ ~ 1.6 · 109 e+ s-1

The loss factor in the spectrometer itself, was found to be consistent for all measurements and amounted to a factor 2.1±0.1, equivalent to a transmission factor of » 48 %. This loss was attributed to the inability to transmit high divergences through the spectrometer. Typical beam spot sizes after the spectrometer were 15 mm horizontally, 55 mm vertically, both FWHM, with divergences of the order of 300 mrad. horizontally and 80 mrad. vertically.

With a 450 micron thick Mylar degrader, as well as an 80 mm diameter CH2-collimator, a total muon rate of only:

Nm+ ~ 6.0 · 107 m+ s-1

could be transmitted up to the spectrometer, whereas a maximum of:

Nm+ ~ 3.2 · 107 m+ s-1

were measured at the final focus, behind the spectrometer. A typical beam spot size at this location was 23 mm horizontally and 68 mm vertically, both FWHM. This shows that there is a total loss factor of about 11.5 between muons entering the area without a degrader and muons reaching the final focus with a degrader. However, the measurements show that already a factor of ~5.5 is lost between the degrader and the entrance to the area, whereas, as mentioned above, a factor ~2.1 is lost in the spectrometer. This dramatic loss of muons in the beamline between the degrader and the entrance to the area could not be improved upon, even when using a smaller diameter collimator and re-tuning. This loss, although not totally understood, is attributed to the increased vertical divergence after the degrader, due to multiple scattering. Horizontally, the divergences are large at the degrader, set by the horizontal focusing properties of the sector magnet AST41, whereas vertically it is only constrained by the inserted CH2-collimator. A muon momentum scan of the kinematic edge of pion decay (29.79 MeV/c), gave the central beam momentum as ~1.3% higher than the assumed value.

1.4.  Results - Separation Quality

From the analysis of data taken while scanning the muon beam spot after the spectrometer, using a small pill-counter mounted on a remotely controlled X-Y table. The different beam constituents could be determined by applying pulse-height and timing cuts to the offline data. The results are shown in Fig. 9Fig. 5Fig. 9Fig. 9. As expected, the suppression of beam positrons is very good, at the peak of the muon distribution the suppression factor is 16.5, with a gently increasing contribution as one goes to higher momenta i.e. lower x-values. Below x = 190 mm the distributions drop again, this is artificial and due to an acceptance cut-off in the spectrometer. The total suppression factor for beam positrons is approximately 90, however, with suitable collimation a factor of more than 300 could be envisaged.

Fig. 959 Offline results from a scan of the muon beam profile, post spectrometer, using a pill-counter. Shown horizontally is the scanner position in millimetres. Vertically the normalized counts are shown. The muon beam profile is shown relative to the contaminant positron distributions from beam positrons (red) and decay positrons i.e. Michel positrons (green). The positron distributions below x = 190 mm are cut-off due to the spectrometer acceptance. High beam momenta correspond to low x-value.

Unfortunately, as is apparent from Fig. 9Fig. 5Fig. 9Fig. 9 there is a large decay positron background present, the origin of this requires further investigation. A part of these Michel positrons come from muon stops in the material close to the scintillator (e.g. holder and PMT) these events should follow the same shape as the muon peak. Since the decay positrons also show the same cut-off behaviour as the beam positrons, it is assumed that the majority come from upstream of the spectrometer. Their source is probably correlated in part to the factor 5.5 loss after the degrader and in part to the factor 2.1 loss in the spectrometer. A contribution from the CH2-collimator, at the degrader location, is also expected. However, this latter effect could not be tested due to the technical problem with the beamline.

1.5.  U/Z-Branch Comparison

Table 1

Table 1

Table 1

Table 1 shows comparative values measured in the "Z"-branch [Ref. 3Ref. 2Ref. 3Ref. 3] and the "U"-branch (present evaluation). The numbers from [Ref. 3Ref. 2Ref. 3Ref. 3] have been scaled to 1800 mA and already constitute the maximum momentum byte and a 6 cm Target E.

Table 1 Comparison of "U"- and "Z"-branches for various conditions

The numbers indicate that without a degrader the muon intensities are the same, whereas in the "U"-branch the beam positron contamination seems to be more than 2.5 times higher than in the "Z"-branch. The beam positron suppression factor is clearly better in the "U"-branch, using a spectrometer. The absolute difference in muon intensities at the final focus is more than a factor of 6, in favour of the "Z"-branch.

1.6.  Preliminary Conclusions

The transmitted surface muon intensities in both of the pE5 branches are equivalent without a degrader, however, the beam positron contamination seems to be more than a factor of 2.5 times higher in the "U"-branch. The use of a spectrometer in this branch causes a loss of about a factor 2.1 in muon intensity, irrespective of condition. This could, in principle, be reduced, by reducing the overall length of the spectrometer such that the higher divergences, which seem to be the ones associated with the loss, would have a better chance of being transmitted.

The situation with a degrader shows a dramatic loss in muon intensity of a factor 5.5, already in front of the spectrometer. This loss, although not totally understood at present, is most likely due to the increased vertical divergence introduced by the multiple scattering of the degrader foil. This is unlikely to be solved in the future. The absolute rate at the final focus compared with the "Z"-branch is more than a factor of 6 lower. The separation quality of beam positrons however, is superior on using a spectrometer.