Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 49 (2016)

FAMUexperiment:
experiment: studies
studies onon thethe muon
muon transfer process
FAMU
processin
inaa
mixtureand higher
higherZZgas
gas
mixture ofof hydrogenhydrogen andA. Vacchi,䠆1,䠆2 E. Mocchiutti,䠆1 D. Guffanti,䠆1, *3 on behalf of the FAMU collaboration
The final objective of the FAMU experiment is to measure
the proton Zemach radius by measuring the hyperfine
VSOLWWLQJRIWKHȝSJURXQGVWDWH1), 2). The experimental methodrequires a detection system suited for time resolved X-ray
spectroscopy. The results of the first measurements
performed at the RIKEN-RAL muon facility are presented inthis paper.
The characteristic X-rays from muonic atoms formed indifferent targets have been detected using a HPGe detector
and five scintillating counters based on LaBr3(Ce) crystals,
whose output was UHFRUGHG IRU  ȝV XVLQJ D  0+]digitizer to measure both energy and time spectrum of thedetected events. With a detailed pulse analysis, considering
also the pile-up events, the expected characteristic X-rays
and lifetimes of various elements were measured.
The measurement of the Zemach radius of the proton Rp
was measured in ordinary hydrogen; therefore, a comparison
with the value extracted from muonic hydrogen may
reinforce or delimit the proton radius puzzle. In the proposedlaser spectroscopy experiment3), 4), muonic hydrogen atoms
are formed in a hydrogen gas target. In subsequent collisions
with H2 molecules, WKHȝSGH-excite to the WKHUPDOL]HGȝSLQ
the (1S) F =0 state. A laser tuned on the HFS resonanceinduces singlet-to-triplet transitions; therefore, the ȝSDWRPVin the (1S) F =1 state are de-excited back to the singlet state
and the transition energy is converted into additional kinetic
HQHUJ\ RI WKH ȝS V\VWHP 7KXV WKH ȝS DWRP JDLQV DERXW
two-thirds RIWKHK\SHUILQHWUDQVLWLRQHQHUJ\ §PH9 
The energy dependence of the muon transfer from muonichydrogen to another higher-Z gas is exploited to detect theoccurred transition LQ ȝS. Although in theory, themuon-transfer rate at low HQHUJLHV ȜS= LV energy
independent, this is not the case for few gases. Oxygen5), 6)
exhibitV D SHDN LQ WKH PXRQ WUDQVIHU UDWH ȜpZepith at theepithermal energy. Thus, by adding small quantities ofoxygen to hydrogen, one can observe the number of HPF
transitions, which take place from the muon-transfer events,
by measuring the time distribution of the characteristic
X-rays of the added gas.
In the first FAMU experimental test, four different targets
were exposed to the muon beam: a pure graphite block andthree gas mixtures (pure H2, H2+2%Ar, H2+4%CO2)
contained in an aluminum vessel. The aim was to study
detector response in the environment of the muon beam atRIKEN-RAL through the measurement of the muon transfer
rate at room temperature.
The characteristic X-rays of muonic atoms were detectedusing scintillating counters based on LaBr3(Ce) crystals
HQHUJ\UHVROXWLRQDWNH9DQGGHFD\WLPHĲ ns) readout by Hamamatsu R11265-200 PMTs and two
HPGe detectors were used to obtain a benchmark spectrum.
Hence, the waveforms were processed off-line to reconstruct
the time and energy of each detected X-ray. By studying thedifferences between the time distribution (see Fig. 1) ofprompt events, represented by X-rays originating IURPȝ$O
atoms formed in the vessel and the delayed X-rays emitted
E\ ȝ2 $U , atoms it was possible to measure the muontransfer rate from hydrogen to oxygen (argon). The results
will be submitted for publication in an international journal.
At present, the collaboration is focussed on the measurementof the temperature dependence of the muon transfer rate by
the mean of the cryogenic
target in order todetermine the best high-Z
gas type, concentrationand temperature. The
experiment is on-goingat RIKEN RAL. Figure 2
shows the top view of theapparatus. Beam pipe ison the left and the target is Fig. 2. FAMU, 2016 layout.
surrounded by the hodoscope, 9 LaBr3(Ce), four HPGe, andthree lutetium aluminate detectors. Measurement of theZemach radius with laser setup will be carried out in 2017.
References

Fig. 1. Time difference between prompt signal (AluminiumX-rays) and delayed signal (Oxygen X-rays).
INFN Trieste and RIKEN Nishina Center
Department Math, Info, Physics, University of Udine
䠆3
INFN GSSI
䠆1䠆21) D. Bakalov, C. Rizzo et al., Phys. Lett. A172 (1993).
2) D. Bakalov, A. Adamczak et al., NIM B281 (2012).
3) D. Bakalov, A. Adamczak et al., Hyperfine Interact. 136-137(2001).
4) D. Bakalov, A. Adamczak et al., Phys. Lett. A379 (2105).
5) F. Mulhauser, H. Schneuwly, Hyperfine Interact. 82 (1993).
6) $:HUWKPXࡇOOHU, A. Adamczak et al., Hyperfine Interact. 116(1998).

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