Andrea Gutierrez

Estimate of the neutron fields in ATLAS based on ATLAS-MPX detectors data

The ATLAS-MPX detectors are based on Medipix2 silicon devices designed by CERN for the detection of different types of radiation. These detectors are covered with converting layers of 6LiF and polyethylene (PE) to increase their sensitivity to thermal and fast neutrons, respectively. These devices allow the measurement of the composition and spectroscopic characteristics of the radiation field in ATLAS, particularly of neutrons. These detectors can operate in low or high preset energy threshold mode. The signature of particles interacting in a ATLAS-MPX detector at low threshold are clusters of adjacent pixels with different size and form depending on their type, energy and incidence angle. The classification of particles into different categories can be done using the geometrical parameters of these clusters. The Medipix analysis framework (MAFalda) — based on the ROOT application — allows the recognition of particle tracks left in ATLAS-MPX devices located at various positions in the ATLAS detector and cavern. The pattern recognition obtained from the application of MAFalda was configured to distinguish the response of neutrons from other radiation. The neutron response at low threshold is characterized by clusters of adjoining pixels (heavy tracks and heavy blobs) left by protons and heavy ions resulting from neutron interactions in the converting layers of the ATLAS-MPX devices. The neutron detection efficiency of ATLAS-MPX devices has been determined by the exposure of two detectors of reference to radionuclide sources of neutrons (252Cf and 241AmBe). With these results, an estimate of the neutrons fields produced at the devices locations during ATLAS operation was done.

Heavy ion charge and velocity resolution with a Medipix-based active Space Radiation Dosimeter

In preparation for the development of an active Space Radiation Dosimeter based on the Medipix2 pixel ASIC technology developed at CERN in Geneva, Switzerland, exposures were made at the HIMAC facility in Japan with a TimePix-based version with a Si detector layer to explore the potential for discrimination between tracks with differing charges and energies, but with very similar dE/dx values.12 Data were taken in February 2009 at 15 degree increments for a number of different beams including 600 and 800 MeV/A Si, 180 MeV/A Ne and 100 MeV/A O. Data were also obtained for 400 MeV/A Si and 500 MeV/A Fe along with 290 and 180 MeV/A N. The TimePix chips have been calibrated to achieve the maximum resolution. One of the principal objectives of these data runs was to explore the resolution of TimePix-based Si detectors to discriminate between various ions with different energies and charges, but with similar dE/dx values in Si. The ongoing analysis is intended to determine the charge and energy resolution capability of the TimePix-based Si detectors. Additional runs were made in September 2009 and preliminary analyses of the data from those runs are included in the results described in this paper. There are more runs planed for January 2010, and preliminary results from those runs should be available for presentation at the conference. The data show an ability to distinguish slow from fast particles as well as a capability to make reasonable estimates of the charge of a traversing particle. The limit of the resolution for such measurements will require additional data, but the clear ability of the technology to measure the LET (Linear Energy Transfer) of traversing charged particles has been shown. The present version of the TimePix ASIC does show evidence of saturation at the highest LET levels, and this information can be used to guide the development of the next generation of the technology.

Study of the charge sharing in silicon pixel detector with heavy ionizing particles interacting with a Medipix2 and a Timepix devices

The charge sharing effect caused by the lateral spread of charge carriers under the influence of the electric field has been investigated in the Medipix2 and Timepix pixel detectors of 300 μm thicknesses exposed to protons and heavier ions, namely alpha-particles, 6Li, 7Li, carbon and oxygen ions.