We demonstrate techniques to improve the accuracy of the energy calibration of Timepix pixel detectors, used for the measurement of energetic particles. The typical signal from such particles spreads among many pixels due to charge sharing effects. As a consequence, the deposited energy in each pixel cannot be reconstructed unless the detector is calibrated, limiting the usability of such signals for calibration. To avoid this shortcoming, we calibrate using low energy X-rays. However, charge sharing effects still occur, resulting in part of the energy being deposited in adjacent pixels and possibly lost. This systematic error in the calibration process results in an error of about 5% in the energy measurements of calibrated devices. We use FLUKA simulations to assess the magnitude of charge sharing effects, allowing a corrected energy calibration to be performed on several Timepix pixel detectors and resulting in substantial improvement in energy deposition measurements. Next, we address shortcomings in calibration associated with the huge range (from kiloelectron-volts to megaelectron-volts) of energy deposited per pixel which result in a nonlinear energy response over the full range. We introduce a new method to characterize the non-linear response of the Timepix detectors at high input energies. We demonstrate improvement using a broad range of particle types and energies, showing that the new method reduces the energy measurement errors, in some cases by more than 90%.

It is demonstrated that modern pixel detectors can be utilized as single-layer particle telescopes, offering details of a particle's stopping power evolution surpassing those provided by multi-layer, non-pixelated instruments. For particles that stop in the detector, this advantage arises from repeatably sampling the Bragg curve: we always know which part of the Bragg peak was measured. We can then create a dE/dx₁ vs dE/dx₂ plot where the stopping power at the beginning and the end of the track is compared. We are able to identify and analyze several fine-grained features on such plots, including several related to particles that stop inside the detector, termed “stopping.” Using data from an instrument aboard the International Space Station, we show that different isotopes of stopping hydrogen can be identified as their stopping powers differ. Other features of the dE/dx₁ vs dE/dx₂ plot not resolvable in multi-layer particle telescopes are also exhibited, such as nuclear interactions that occur within the sensor active volume.

We report on the status of LAMPF experiment E-1240 to measure the Michel ρ parameter in direct muon decay. This experiment ran in 1993, and the data are currently being analyzed. The expected precision on the ρ parameter is $\text{±0.0008.}$ This result will provide better constraints on new physics, particularly on the charged vector bosons’ mixing angle ζ in the manifestly left-right symmetric extension of the Standard Model.

A new measurement of the Michel parameter ρ in normal muon decay has been performed using the MEGA positron spectrometer. Over 500 million triggers were recorded and the data are currently being analyzed. The previous result has a precision on the value of ρ±0.0026. The present experiment expects to improve the precision to ±0.0008 or better. The improved result will be a precise test of the standard model of electroweak interactions for a purely leptonic process. It also will provide a better constraint on the W_R−W_L mixing angle in the left‐right symmetric models.

The MEGA experiment is designed to search for the rare decay μ→eγ with a branching ratio sensitivity of ∼5×10⁻¹³. Production data have been taken during 1992 and 1993, and the detector is working as expected. Following a complete analysis, the present data set should represent an improvement of 12–15 in sensitivity over the previous limit of μ→eγ.