Abstract
The FAMU experiment aims to accurately measure the hyperfine splitting of the ground state of the muonic hydrogen atom. A measurement of the transfer rate of muons from hydrogen to heavier gases is necessary for this purpose. In June 2014, within a preliminary experiment, a pressurized gas-target was exposed to the pulsed low-energy muon beam at the RIKEN RAL muon facility (Rutherford Appleton Laboratory, U.K.). The main goal of the test was the characterization of both the noise induced by the pulsed beam and the X-ray detectors. The apparatus, to some extent rudimental, has served admirably to this task. Technical results have been published that prove the validity of the choices made and pave the way for the next steps. This paper presents the results of physical relevance of measurements of the muon transfer rate to carbon dioxide, oxygen, and argon from non-thermalized excited μp atoms. The analysis methodology and the approach to the systematics errors are useful for the subsequent study of the transfer rate as function of the kinetic energy of the μp currently under way.
Highlights
Background subtractionIn order to obtain the number of X-ray events for each time bin, it is necessary to subtract the background below the X-ray carbon, oxygen, and aluminium lines
This paper presents the results of physical relevance of measurements of the muon transfer rate to carbon dioxide, oxygen, and argon from non-thermalized excited μp atoms
The difference between the distributions obtained from the simulation, figure 2 on the right, can not account for the time difference of about 5–10 ns observed in the real data between aluminium and CO2 X-rays distributions. This implies that a physical process like the muon transfer from muonic hydrogen to CO2 and argon is responsible for this time difference
Summary
At RAL muons are produced in bunches with a repetition rate of 50 Hz. Each bunch is consisting of two spills separated by about 320 ns. The initial momentum was chosen in order to maximize the muon stop in the gas of the target. In the case of light elements, de-excitation starts with Auger process until the quantum number reaches values from 3 to 6 at which radiative transitions take over. In presence of material impurities with mass number A>1 and atomic number Z>1, the muonic hydrogen system can undergo the transfer reaction μ−p + AZ → (μ− AZ)∗ + p, which results in the formation of a new muonic atom (μ− AZ)∗ which de-excites by Auger and radiative processes. The target was evacuated at a vacuum level below 10−5 mbar. This procedure assured a contamination level smaller than one part per million during the data taking.
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