Muons-On-REquest (MORE) -- piM3 Area

(A full description of the MORE setup is given in Ref. [1])

Introduction

In conventional time-differential µSR experiments, an ideal lifetime spectrum would contain only start-stop events belonging to the same muon. In real experiments using continuous muon beams, however, there is always a certain number of muons escaping detection, e.g. those stopping near but not hitting the muon counter. This results in a random rate of events in the start and stop channels and a random background in the muon-decay histograms, thus limiting the useful time interval to about 10 µs and excluding investigations of low muon-spin precession frequencies or slow relaxation processes.

Pulsed muon beams, on the other hand, deliver many muons per pulse at low repetition rates (e.g. 50Hz at ISIS). In this technique, the time resolution is limited by the finite length of the muon pulses (80ns at ISIS compared to 1ns on the GPS instrument at PSI). However, the background is typically three orders of magnitude lower.

A method to solve the background problem at continuous muon beams has been proposed earlier (see [2] or [3]) for surface-muon beams at TRIUMF or KAON but has never been realised. The basic idea is to extract only one muon at a time out of a continuous beam by means of a fast-switching electrostatic deflector ("kicker") on request from a µSR instrument. This makes sure that no other muon reaches the spectrometer until the extracted one has been processed. Moreover, the concept of "Muons-On-REquest" (MORE) does not significantly reduce the intensity of the original beam which is therefore available for the simultaneous use by a second spectrometer, i.e. MORE produces twice as many results of even higher quality (lower background in one spectrometer) than the conventional technique.
 

Experimental Setup in the piM3 Area

The main components installed for MORE in the surface-muon beam line piM3 at PSI are shown in the Figure 1 together with the two instruments GPS (General Purpose Surface-muon spectrometer) and LTF (Low Temperature Facility).
 

Figure 1: Layout of the piM3 surface-muon beam line at PSI for the muons-on-request (MORE) technique.
 

The kicker contains two 1m long, 20cm wide electrodes 20cm apart (see Figure 2). Two power supplies for dc voltages up to +5kV and -5kV  are connected to the electrodes via fast switches, giving a difference of 20kV between the two field directions and a separation of the muon trajectories of about 5cm at the intermediate beam focus in front of the septum magnet located about 5m from the kicker exit. Each switch consists of a series of 15 high-voltage MOSFET transistors type IXYS 6N100.
 
 

Figure 2 :
(a) Schematic circuit of the fast-switching deflector ("kicker") for muons on request. Power per switch <600VA, repetition rate <40kHz, voltage |U|<10kV between electrodes, switching time: <25ns (10-90%).
(b) Driving circuit for one switch. Delay between trigger signal input and high-voltage output ca. 40ns.

The muon detector (M-counter) in the spectrometer (GPS or LTF) is used to trigger the kicker. The kicker is switched to the spectrometer running in "MORE mode" (say, GPS) for a maximum of 5µs at a fixed repetition rate (max. 40kHz). The signal of the first muon hitting the trigger detector (M-counter) after a minimum delay of 200ns is used to switch the kicker back to the spectrometer running in "parasitic mode" (LTF in this case). The delay is necessary to avoid damage to the power switches.

Either instrument, GPS or LTF, can be used in MORE mode while the other one is running in "parasitic" mode. Two variable slits (FS301 and FS302 for LTF and GPS, respectively) and located in the intermediate focus plane of the beam in front of the septum magnet are used to adjust the event rate in both legs individually. The two instruments can also be used simultaneously in conventional "shared" mode by means of a horizontally defocused beam spot in the septum magnet slit plane.
 

Results

Figure 3 shows an example of µSR in silver in an external magnetic field of 10mT, taken with the GPS in MORE mode.  For comparison a conventional spectrum is shown taken at the same event rate. The background in MORE mode is at least a factor of 100 lower than in conventional mode, thus allowing the study of muon-spin precession and relaxation easily up to 20µs.
 

Figure 3 : µSR in silver in a magnetic field of 10mT measured at the GPS facility instrument in conventional and in MORE mode.
Insert: Reduced asymmetry plot for the first 2µs in MORE mode (function fitted for t > 0.4µs).
 

	Conventional	MORE	Pulsed µSR
Trigger	none	GPS	50 Hz
B0/N0 [10-5]	660	8.7	ca. 1
Time resol. [ns]	1	1	80
Event. rate [106/h]	12	20	10-20

Table 1 : Comparison of results obtained with GPS in conventional and in MORE mode (using the GPS M-counter as trigger). Values for pulsed µSR (ISIS) are also shown.

In MORE mode with M-counter trigger we observe a small distortion in the spectra at times t < 450ns (insert in Figure 3) which is due to the delay between the passage of the muon and the arrival of the trigger signal at the kicker.During this time, additional muons can enter the spectrometer and "kill" events through pile-up rejection.  Fitting the distorted spectra is possible using different values of the fit parameters N₀ (normalizing constant) and B₀ (background) in the two regions. However, the MORE technique is preferentially used for slow signals where cutting off the first channels does not affect the data analysis.

The unique new feature of the system is that - due to the high time resolution - it opens the possibility of studying slow relaxation phenomena at high magnetic fields and to resolve close spin-precession frequencies (see the striking example obtained in UPt₃ [4]).
 

References

1. R. Abela, A. Amato, C. Baines, X. Donath, R. Erne, D.C. George, D. Herlach, G. Irminger, I.D. Reid, D. Renker, G. Solt, D. Suhi, M. Werner and U. Zimmermann,
   Hyperfine Interact. 120/121 (1999) 575.
2. J.H. Brewer, 
   Hyperfine Interact. 66 (1990) 1137.
3. J.L. Beveridge, 
   Z. Phys. C 56 (1992) S258.
4. A. Yaouanc, P. Dalmas de Réotier, F. N. Gygax, A. Schenck, A. Amato, C. Baines, P. C. M. Gubbens, C. T. Kaiser, A. de Visser, R. J. Keizer, A. Huxley, and A. A. Menovsky,
   Phys. Rev. Lett. 84 (2000) 2702-2705.