Beam Extinction Monitoring In The Mu2e Experiment

The Mu2e Experiment at Fermilab will search for the conversion of a muon to an electron in the field of an atomic nucleus with unprecedented sensitivity. The experiment requires a beam consisting of proton bunches approximately 200ns FW long, separated by 1.7 microseconds, with no out-of-time protons at the 10−10 fractional level. The verification of this level of extinction is very challenging. The proposed technique uses a special purpose spectrometer which will observe particles scattered from the production target of the experiment. The acceptance will be limited such that there will be no saturation effects from the in-time beam. The precise level and profile of the out-of-time beam can then be built up statistically, by integrating over many bunches.

The proposed Mu2e experiment at the future muon campus at the Fermi National Accelerator Laboratory is designed to probe for new physics (in particular, muon to electron conversion) at the intensity frontier. Mu2e is designed to observe and measure charged lepton flavor violation (CLFV) via muon capture (u ̄ + N(Z) [arrow right] e ̄+ N(Z)) at an event sensitivity of approximately 5.4 x 10−17 on a 90% confidence interval. Achieving this level of sensitivity necessitates a beam extinction monitor to detect secondary particles indicative of out of time (OoT) beam protons. If the beam extinction exceeds the design value of 10−10, prompt background levels will not be adequately suppressed to achieve the desired event sensitivity. A series of Monte Carlo simulations were performed to study the extinction monitor's ability to record OoT events as well as its sensitivity to misalignment during the construction of the experiment. Additionally, various extinction monitor geometry changes were implemented within Geant4 to measure and minimize the effects of damaging radiation on crucial detector elements. Independent analysis of these simulations indicate the nominal extinction monitor design meets the Mu2e experiment's requirements. Minimal design changes substantially mitigate the effects of harmful radiation damage and provide for a more robust detector-maximizing the extinction monitor's lifetime with hardware replacement on the order of approximately once a year, while minimizing operational cost and experiment downtime.

For future experiments at the intensity frontier, precise and accurate knowledge of beam time structure will be critical to understanding backgrounds. The proposed Mu2e experiment will utilize ≈200 ns (FW) bunches of 3 x 107 protons at 8 GeV with a bunch-to-bunch period of 1695 ns. The out-of-bunch beam must be suppressed by a factor of 10−1° relative to in-bunch beam and continuously monitored. I propose a Cerenkov-based particle telescope to measure secondary production from beam interactions in a several tens of microns thick foil. Correlating timing information with beam passage will allow the determination of relative beam intensity to arbitrary precision given a sufficiently long integration time. The goal is to verify out-of-bunch extinction to the level 10−6 in the span of several seconds. This will allow near real-time monitoring of the initial extinction of the beam resonantly extracted from Fermilabs Debuncher before a system of AC dipoles and collimators, which will provide the final extinction. The effect on beam emittance is minimal, allowing the necessary continuous measurement. I will present the detector design and some concerns about bunch growth during the resonant extraction.

Mu2e is a planned experiment to search for flavor violating conversion from a muon to an electron. The experiment will use a pulsed 8 GeV proton beam to produce muons which will then stop in an aluminum target. Mu2e will search for the $\mu^- + Al \rightarrow e^- + Al$ process. For Mu2e, an extinction rate of 10$^{-10}$ is required to reduce the backgrounds to an acceptable level. Extinction is the ratio of the amount of protons striking the production target between beam pulses to the number striking it during the beam pulse. One of the backgrounds, off-target interactions, was simulated using G4beamline and Fermilab's Grid setup to confirm that an extinction rate of 10$^{-10}$ is possible. The extinction level will be measured by the extinction monitor which will include scintillation counters read out by photomultiplier tubes. In order to build a beam time profile, low fake responses (after pulses) are needed in the photomultiplier tubes. This thesis determines the best co mbination of resistors, voltage, and other components that provide the lowest after pulse rate.

Mu2e is a planned experiment to search for flavor violating conversion from a muon to an electron. The experiment will use a pulsed 8 GeV proton beam to produce muons which will then stop in an aluminum target. Mu2e will search for the mu-- + Al → e-- + Al process. For Mu2e, an extinction rate of 10--10 is required to reduce the backgrounds to an acceptable level. Extinction is the ratio of the amount of protons striking the production target between beam pulses to the number striking it during the beam pulse. One of the backgrounds, off-target interactions, was simulated using G4beamline and Fermilab's Grid setup to confirm that an extinction rate of 10--10 is possible. The extinction level will be measured by the extinction monitor which will include scintillation counters read out by photomultiplier tubes. In order to build a beam time profile, low fake responses (after pulses) are needed in the photomultiplier tubes. This thesis determines the best combination of resistors, voltage, and other components that provide the lowest after pulse rate.

For future experiments at the intensity frontier precise and accurate knowledge of beam time structure will be critical to understanding backgrounds. The proposed Mu2e experiment calls for ≈ 200ns (FW, 40 ns rms) bunches of 3 x 107 8 GeV protons and a bunch spacing of 1695 ns. The interbunch beam must be suppressed from the main pulse by a factor of 10−1°, this is known as the beam extinction requirement. Beam from Fermilab's Booster will be formed into 2.5 MHz buckets in the Fermilab Recycler then transferred to the Delivery Ring (formerly the Debuncher) and slow spilled from a single filled bucket in an h = 4 RF system. Because the final extinction level is not expected from the Delivery Ring an AC dipole and collimation system will be used to achieve final extinction. Here I present calculations leading to a first estimate of the extinction level expected upon extraction from the Delivery Ring of ≤ 3.36 x 10−4. Intrabunch, residual gas scattering and scattering off the extraction septum are included. Contributions from bunch formation are not considered.

A muon-to-electron conversion experiment at Fermilab, Mu2e, is being designed to probe for new physics beyond the standard model at mass scales up to 104 TeV. For this experiment, the advance in experimental sensitivity will be four orders of magnitude when compared to existing data on charged lepton flavor violation. The muon beam will be produced by delivering a proton beam contained in short 100-ns bunches onto a muon production target, with an inter-bunch separation of about 1700 ns. A critical requirement of the experiment is to ensure a low level of background at the muon detector consistent with the required sensitivity. To meet the sensitivity requirement, protons that reach the target between bunches must be suppressed by an enormous factor, so that an extinction factor, defined as a number of background protons between main bunches per proton in such a bunch, should not exceed 10−9. This paper describes the advanced beam optics and results of numerical modeling with STRUCT and MARS codes for a beam line with a collimation system that allows us to achieve the experimental extinction factor of one per billion.

The Mu2e experiment is being planned at Fermilab to measure the rate for muons to convert to electrons in the field of an atomic nucleus with unprecedented precision. This experiment uses an 8 GeV primary proton beam consisting of short (H"00 nsec FW) bunches, separated by 1.7?sec. It is vital that out-of-bunch beam be suppressed at the level of 10−1° or less. This poster describes the parametric analysis which was done to determine the optimum harmonics and magnet specifications for this system, as well as the implications for the beam line optics.