FAMU Experiment Research Articles

The 2024 LaBr[formula omitted](Ce) detector setup for the FAMU experiment

The FAMU experiment, led by INFN at the Rutherford Appleton Laboratory (UK), is designed to measure the Zemach radius of the proton in muonic hydrogen (μp) with uncertainty better than 1%. A set of LaBr3(Ce) scintillating crystals, complemented by a High-Purity Germanium (HPGe) detector, are used to detect the delayed muonic oxygen (μO) X-ray emission, which signals the muonic hydrogen spin flip excited with a tunable MIR laser. The detector setup is in continuous development. After two periods of measurement in 2023, the detector configuration is currently being upgraded for the 2024–2025 runs with new detectors read out by SiPMs to enhance signal-to-noise ratio and improve time performance. In particular, all ½” scintillators are being substituted with 1” ones, increasing the solid angle coverage and optimising energy resolution and timing performances.

The muon beam monitor for the FAMU experiment: design, simulation, test, and operation

FAMU is an INFN-led muonic atom physics experiment based at the RIKEN-RAL muon facility at the ISIS Neutron and Muon Source (United Kingdom). The aim of FAMU is to measure the hyperfine splitting in muonic hydrogen to determine the value of the proton Zemach radius with an accuracy better than 1%. The experiment has a scintillating-fibre hodoscope for beam monitoring and data normalisation. In order to carry out muon flux estimation, low-rate measurements were performed to extract the single-muon average deposited charge. Then, detector simulation in Geant4 and FLUKA allowed a thorough understanding of the single-muon response function, which is crucial for determining the muon flux. This work presents the design features of the FAMU beam monitor, along with the simulation and absolute calibration measurements in order to enable flux determination and enable data normalisation.

Status of the detector setup for the FAMU experiment at RIKEN-RAL for a precision measurement of the Zemach radius of the proton in muonic hydrogen

The FAMU experiment at RIKEN-RAL is a muonic atom experiment with the aim to determine the Zemach radius of the proton by measuring the 1s hyperfine splitting in muonic hydrogen. The activity of the FAMU Collaboration in the years 2015–2023 enabled the final optimisation of the detector-target setup as well as the gas working condition in terms of temperature, pressure and gas mixture composition. The experiment has started its data taking in July 2023. The status of the detector setup for the 2023 experimental runs, for the beam characterisation and muonic X-ray detection in the 100–200 keV energy range, is presented and discussed.

Characterisation of a low-momentum high-rate muon beam monitor for the FAMU experiment at the CNAO-XPR beam facility

The FAMU experiment aims at an indirect measurement of the Zemach radius of the proton. The measurement is carried out on muonic hydrogen atoms (μH) produced through the low-momentum (50–60 MeV/c) muon beam a the RIKEN-RAL μ- facility. The particle flux plays an important role in this measurement, as it is proportional to the number of μH atoms produced, which is the target of the FAMU experimental method. The beam monitor calibration technique and results, presented here, are meant to extract a reliable estimation of the muon flux during the FAMU data taking. These measurements were carried out at the CNAO synchrotron in Pavia, Italy, using proton beams and supported by Monte Carlo simulation of the detector in Geant4.

Improving the Time Resolution of Large-Area LaBr3:Ce Detectors with SiPM Array Readout

LaBr3:Ce crystals have good scintillation properties for X-ray spectroscopy. Initially, they were introduced for radiation imaging in medical physics with either a photomultiplier or SiPM readout, and they found extensive applications in homeland security and gamma-ray astronomy. We used 1″ round LaBr3:Ce crystals to realize compact detectors with the SiPM array readout. The aim was a good energy resolution and a fast time response to detect low-energy X-rays around 100 keV. A natural application was found inside the FAMU experiment, at RIKEN RAL. Its aim is a precise measurement of the proton Zemach radius with impinging muons, to contribute to the solution to the so-called “proton radius puzzle”. Signals to be detected are characteristic X-rays around 130 KeV. A limit for this type of detector, as compared to the ones with a photomultiplier readout, is its poorer timing characteristics due to the large capacity of the SiPM arrays used. In particular, long signal falltimes are a problem in experiments such as FAMU, where a “prompt” background component must be separated from a “delayed” one (after 600 ns) in the signal X-rays to be detected. Dedicated studies were pursued to improve the timing characteristics of the used detectors, starting from hybrid ganging of SiPM cells; then developing a suitable zero pole circuit with a parallel ganging, where an increased overvoltage for the SiPM array was used to compensate for the signal decrease; and finally designing ad hoc electronics to split the 1″ detector’s SiPM array into four quadrants, thus reducing the involved capacitances. The aim was to improve the detectors’ timing characteristics, especially falltime, while keeping a good FWHM energy resolution for low-energy X-ray detection.

Characterisation of a scintillating fibre-based hodoscope exposed to the CNAO low-energy proton beam

A 32 × 32 Bicron 1 mm2 polystyrene scintillating fibre-based beam hodoscope, with an entrance window of 6.3 × 6.3 cm2, has been designed and characterised for monitoring low-energy charged particle beams. The hodoscope, designed to fit into the 60 MeV/c negative muon beam at the RIKEN-RAL muon facility (UK) for the FAMU experiment, will enable muon counting for data normalisation. We describe a calibration technique to enable the detector to work as a muon stream counter by knowing the muon equivalent charge deposited in the detector. This value is obtained by exposing the detector to a single-proton beam at the CNAO hadron therapy synchrotron, with the energy set to have the same energy loss profile as 60 MeV/c negative muons.

Characterisation of muon and proton beam monitors based on scintillating fibres with a SiPM read-out

Beam hodoscopes based on 32 × 32 square polystyrene scintillating fibres along X/Y, having section 3 × 3 mm2 or 1 × 1 mm2, have been designed and characterised for monitoring low-energy charged particle beams. Each fibre is read by a SiPM and the signal is digitised and analysed off-line. Hodoscopes were exposed to the 60 MeV/c negative muon beam at Port 1 of the RIKEN-RAL muon facility (UK), where they proved to be good to determine the beam profile and its intensity, via a calibration with cosmic muons. Furthermore, the exposition to the calibrated single-proton beam at CNAO (Italy), in the momentum range between 340 MeV/c and 690 MeV/c, provided further calibration in dE/dx and showed the feasibility of the detector as an instrument for proton beam characterisation too. These hodoscopes will be used as negative muon beam monitors for the FAMU experiment at the Rutherford Appleton Laboratory, but the possibility of using them as a beam monitor in hadron therapy has also been investigated.

Large area LaBr3:Ce crystals read by SiPM arrays with improved timing and temperature gain drift control

Compact X-rays detectors made of 1/2” or 1” :LaBr3:Ce crystals of cubic shape with SiPM array readout have been developed for the FAMU experiment at RIKEN-RAL. The aim is a precise measurement of the proton Zemach radius with incoming muons. Additional applications may be found in medical physics, such as PET, homeland security and gamma-ray astronomy. Due to the high photon yield of LaBr3:Ce it was possible to use a simple readout scheme based on CAEN V1730 digitizers. Detectors, using Hamamatsu S13361 or S14161 arrays, have good FWHM energy resolutions up to 3% (8%) at the Cs137 (Co57) peak, comparing well with the best results obtained with a photomultiplier’s readout. Detailed studies were performed to correct online the drift with temperature of SiPM gain and to reduce the risetime/falltime of detectors’ signals, that increased going from 1/2” to 1” detectors, due to the larger capacity of the used SiPM arrays.

The FAMU experiment: muonic hydrogen high precision spectroscopy studies

The FAMU experiment aims to measure for the first time the hyperfine splitting of the muonic hydrogen ground state. From this measurement the proton Zemach radius can be derived and this will shed light on the determination of the proton charge radius. In this paper, we describe the scientific goal, the method and the detailed preparatory work. This includes the outcome of preliminary measurements, subsequent refined simulations and the evaluation of the expected results. The experimental setup being built for the measurement of the hyperfine splitting to be performed at the RAL laboratory muon facility is also described.

Ce:LaBr3 crystals with SiPM array readout and temperature control for the FAMU experiment at RAL

Compact X-rays detectors made of 1/2′′ Ce:LaBr3 crystals of cubic shape with SiPM array readout have been developed for the FAMU experiment at RIKEN-RAL, to instrument regions of difficult access. Due to the high photon yield of Ce:LaBr3 it was possible to use a simple readout scheme based on CAEN V1730 digitizers, without a dedicated amplification stage. The drift with temperature of SiPM gain was corrected by using CAEN A7885D regulated power supply chips with temperature feedback. Energy resolutions (FWHM) around 3.5% at the 137Cs peak and around 9% at the 57Co peak were obtained.

FAMU: study of the energy dependent transfer rate Λμp → μO

The main goal of the FAMU experiment is the measurement of the hyperfine splitting (hfs) in the 1S state of muonic hydrogen ΔEhfs (μ-p)1S. The physical process behind this experiment is the following: μp are formed in a mixture of hydrogen and a higher-Z gas. When absorbing a photon at resonance-energy ΔEhfs ≈ 0.182 eV, in subsequent collisions with the surrounding H2 molecules, the μp is quickly de-excited and accelerated by ∼ 2/3 of the excitation energy. The observable is the time distribution of the K-lines X-rays emitted from the μZ formed by muon transfer (μp) + Z → (μZ)* + p, a reaction whose rate depends on the μp kinetic energy. The maximal response, to the tuned laser wavelength, of the time distribution of X-ray from K-lines of the (μZ)* cascade indicate the resonance. During the preparatory phase of the FAMU experiment, several measurements have been performed both to validate the methodology and to prepare the best configuration of target and detectors for the spectroscopic measurement. We present here the crucial study of the energy dependence of the transfer rate from muonic hydrogen to oxygen (Λμp → μ0), precisely measured for the first time.

Performance of X-rays crystal detectors with SiPM array readout exposed to the RIKEN-RAL low energy muon beam

Scintillating crystals coupled to SiPM array readout have been proposed to realize compact X-rays detectors for the FAMU experiment, which 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. Since 2014, in a series of test runs, 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 these tests was the characterization of both the background induced by the pulsed beam and the X-ray detectors. Preliminary results are shown on the use of new Pr:LuAG and Ce:GAGG crystals, developed for PET scanners and of more conventional LaBr 3 :Ce crystals, with a SiPM array readout. Energy spectra of the X-rays produced after the de-excitation of muonic atoms formed from the interaction of the beam muons with the gas target in the FAMU apparatus are shown, demonstrating the suitability of these detectors for muonic X-rays spectroscopy.

The upgraded beam monitor system of the FAMU experiment at RIKEN–RAL

A set of three X/Y beam hodoscopes based on square Bicron BCF12 scintillating fibers read by SiPM has been developed for the FAMU experiment at RIKEN–RAL. A beam width (RMS) less than 10 mm is measured in both X/Y directions. Muon rates are measured and found compatible with previous data.

First FAMU observation of muon transfer from μp atoms to higher-Z elements

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.

Toward the measurement of the hyperfine structure of muonic hydrogen in the FAMU experiment: Multi-pass cavity optimization for experiments with pulsed sources.

We consider a simplified model of the optical multi-pass cavity that is being currently developed by the FAMU collaboration for the measurement of the hyperfine splitting in the ground state of muonic hydrogen and of the Zemach radius of the proton. The model is focused on the time distribution of the events of laser-stimulated hyperfine transitions in the muonuc atom and may be helpful in the preliminary design of the FAMU experimental set-up and, more generally, in the optimization of multi-pass optical cavities for experiments with pulsed lasers.

The LaBr3(Ce) based detection system for the FAMU experiment

The FAMU experiment is devoted to the high precision measurement of the hyperfine splitting of the muonic-hydrogen atom ground state using a pulsed and intense muon beam; it requires careful technological choices both in the construction of the gas target and of the detectors. In the last three years, the cryogenic gas target was developed and used for experimental measurement sessions, with the low energy pulsed muon beam (55–65 MeV), at the RIKEN-RAL (U.K.) muon facility. At the same time, eight LaBr3(Ce) based detectors have been developed to be accommodated in a circular support surrounding the target. The readout for each detector consists of a photomultiplier tube (PMT) equipped with an improved fully active high voltage divider and a Digital Pulse Processor (DPP) performing noise reduction with an optimal (triangular) filter. The active voltage divider, together with the DPP, allow to operate at high speed while retaining all the spectrometric properties of the LaBr3(Ce) scintillator. In this paper, the development of this innovative high-performance detection system is presented.

Steps towards the hyperfine splitting measurement of the muonic hydrogen ground state: pulsed muon beam and detection system characterization

The high precision measurement of the hyperfine splitting of the muonic-hydrogen atom ground state with pulsed and intense muon beam requires careful technological choices both in the construction of a gas target and of the detectors. In June 2014, the pressurized gas target of the FAMU experiment was exposed to the low energy pulsed muon beam at the RIKEN RAL muon facility. The objectives of the test were the characterization of the target, the hodoscope and the X-ray detectors. The apparatus consisted of a beam hodoscope and X-rays detectors made with high purity Germanium and Lanthanum Bromide crystals. In this paper the experimental setup is described and the results of the detector characterization are presented.

The FAMU experiment: muonic atoms to probe the proton structure

The goal of the FAMU experiment is the measurement of the proton Zemach radius using muonic hydrogen, a subject that has raised much interest in recent years due to its implications in the so-called proton radius puzzle. In order to extract the Zemach radius, the FAMU collaboration aims at measuring the hyperfine splitting of the µp ground state, since the effect of the proton finite size affects the HF transition energy. The proposed experimental method requires a detection system which is suited for time resolved X-ray spectroscopy: in this contribution the results of the first measurements performed at the RIKEN-RAL muon facility in order to verify the fitness of the detection system in the pulsed intense muon beam are presented. The characteristic X-rays from atomic transitions in muonic atoms formed in different targets have been detected using a HPGe detector and five scintillating counters based on LaBr3(Ce) crystals, whose output has been recorded for 5 ps using a 500 MHz digitizer to measure both the energy and the time spectrum of the detected events. With a detailed pulse analysis considering pile-up events, both the expected characteristic X-rays and lifetimes of various elements were measured, paving the way for future measurements to be carried out in early 2016.

Characterization of Pr:LuAG scintillating crystals for X-ray spectroscopy

The main features of the Pr doped Lu3Al5O12 (Pr:LuAG) scintillating crystals for X-ray spectroscopy applications have been studied using different radioactive sources and photo-detectors. Pr:LuAG is cheaper, compared to a Germanium detector, but with remarkable properties which make it useful for many applications, from fundamental physics measurements to the PET imaging for medical purposes: high density, elevate light yield, fast response, high energy resolution, no hygroscopicity. A sample of Pr:LuAG crystals with 14mm×14mm surface area and 13mm thickness and a NaI crystal of the same surface and 26mm thickness used as a reference have been characterized with several radioactive sources, emitting photons in the range 100–1000keV. Different light detectors were adopted for the Pr:LuAG studies, sensitive to its UV emission (peak at 310nm): a 3in. PMT (Hamamatsu R11065) and new arrays of Hamamatsu SiPM S13361, with siliconic resin as a window. Preliminary results are presented on the performance of the Pr:LuAG crystals, to be mounted in a 2×2 array to be tested in the 2015 run of the FAMU experiment at RIKEN-RAL muon facility. The goal is the detection of the X-rays (around 130keV) emitted during the de-excitation processes of the muonic hydrogen after the excitation with an IR laser with wavelength set at the resonance of the hyperfine splitting, to measure the muonic atom proton radius with unprecedented precision.