MLF

実験報告書様式（一般利用課題・成果公開利用）
提出日 Date of Report
Experimental Report
24th May 2010

課題番号 Project No.
2009A0023
実験課題名 Title of experiment
Measurements of an Extinction Ratio and a Muonic AtomFormation Rate
実験責任者名 Name of principal investigator
Masaharu Aoki
所属 Affiliation
Osaka University

装置責任者 Name of responsible person
Naritoshi Kawamura
装置名 Name of Instrument/（BL No.）D2
実施日 Date of Experiment
2009/6/2 10:00 ~ 22:00
2009/6/17 10:00 ~ 6/18 10:00
2009/11/17 12:30 ~ 11/19 7:00

試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
N/A

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results. If you failed to conduct experiment as planned, please describe reasons.
The experiment was performed at D2 beam line in the muon facility of J-PARC MLF. The polarity of the D2
beam line was set for the negative charged particles. A simple counter assembly was installed at the exit of
the D2 beam line as shown in Fig. 1. Negative muons below 60 MeV/c were all stopped in the Lead absorberplate, and absorbed via nuclear capture process with the lifetime less than 100 nsec. The detection
efficiency for electrons is 50% due to Bremsstrahlung in the Lead plate.
The purpose of this measurement is to estimate the yield of delayed electrons coming through the D2 beamline from the muon production target. In order to make detectors work during the delayed timing, they should
be protected against the prompt burst of the beam coming in coincidence with the primary proton pulse timing.
In this measurement, this issue was solved by the following methods:
1) Read the fast scintillation light from plastic scintillators by photomultipliers (PMT),
2) Apply “gating” to the PMT to effectively switch-off the PMT during the prompt timing.
Figure 2 shows typical signals from each counter. In the “on” state, the shape of the signal baseline is notflat but shows monotonically decreasing offset due to the delayed fluorescence. However, individual signals
are easily recognizable. In order to pick up the timing of these individual pulses, the whole waveform wasrecorded by using flash ADC. Those waveforms were analyzed afterwards by software to find individual
signals on the baseline, and the coincidence between B1 and B2 was taken. Figure 3 shows time spectrum of1

2. 実験方法及び結果（つづき） Experimental method and results (continued)
the coincidence. Exponential shape of the spectrum is seen. The decay constant of the spectrum is 2.10
±0.02 μsec, and that is very closed to the lifetime ofμ- in carbon (2.00 μsec). The cause of small difference
is currently under study.
After establishing the method to measure the yield of the delayed electron yield as described above, the yielddependence on the D2 beam line momentum setting was measured as shown in Fig. 4. The momentum of theedge of the spectrum is about 50 MeV/c, which is consistent with the edge of Michel spectrum of muon decay.
Based on these observations in time spectrum and momentum spectrum, we concluded that the delayedelectrons we observed at the exit of D2 beam line was coming from the muonic carbon atom in the muontarget.
The muonic carbon yield at the muon target was obtained from the above delayed electron yield withcorrection from the D2 beam line acceptance. The D2 acceptance was estimated by Monte Carlo calculation(G4Beamline). The muonic carbon yield at the muon target is 109/sec for 120 kW proton beam power. This
yield is consistent with the Monte Carlo calculated estimation.
From this measurement, we successfully obtained very important information about the muonic carbon yield.
However, we still have to complete the following study before the publication:1) Understand the cause of decay constant being slightly off from 2.0 μsec,
2) Estimate systematic errors.

Fig. 2 Typical signals from counters,
yellow:B1, blue:B2, purple:B3.

Fig. 1 Counter Setup

Fig. 4 Momentum spectrum of delayedelectron.

Fig. 3 Time spectrum of electron.

2