Test of thin Ultra-fast Silicon detectors (UFSD) for monitoring of high flux charged particle beams

Test of thin Ultra-Fast Silicon Detectors (UFSD) for monitoring of high flux charged particle beams.

V.Monaco

1 Università di Torino, via P. Giuria,1, 10125 Torino, Italy 2 INFN, Sezione di Torino, via P. Giuria,1, 10125 Torino, Italy

3 Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy 4 Faculty of Physics, Yazd University, Yazd, Iran

5 Fondazione CNAO, Strada Campeggi 53, 27100 Pavia, Italy

6 Università del Piemonte Orientale, Largo Dobegani 2/3, 28100 Novara, Italy

Abstract

Prototype beam monitoring devices are under development at the Torino section of INFN, with the goal of measuring the flux, the profile and the energy of charged particle beams, in particular for radiobiology and hadron-therapy applications. The devices will employ Ultra-Fast Silicon Detectors (UFSD), innovative silicon sensors optimized for timing measurements based on the Low-Gain Avalanche Diode technology (LGAD), where a controlled internal gain is obtained with an additional p++ layer implanted in a n-on-p silicon diode.

The flux measurement exploits the fast collection time of thin silicon sensors (1 ns in 50 um), which allows to discriminate single beam particles and count their number up to high fluxes, depending on the sensor segmentation. The number of crossing particles will provide a direct measurement of the beam flux independent of the particle energy or other parameters, in contrast to measurements based, for example, on the total charge produced in a gas detector. The beam energy will be determined exploiting the timing capabilities of UFSDs, and using the time of flight information from a telescope of two silicon detectors placed at a distance of less than 1 m along the beam direction.

Dedicated UFSD strip sensors with 50 um thickness have been produced at the Fondazione Bruno Kessler (FBK) in Trento, Italy, and are currently studied in laboratory and in beam tests. The segmentation of the sensors (2

mm2 _{strip area) will allow to measure beam rates up to 10}8 _{Hz/cm^2 with pile-up probability < 1 %. The sensors}

have been produced with different doping alternatives to study the best options in terms of radiation resistance. The enhanced Signal-to-Noise ratio of the UFSD sensors has driven the design of dedicated readout

ASICs able to deal with high signal frequencies (several 108 _{Hz/channel), with two different architectures based}

respectively on a transimpedence amplifier and a charge sensitive amplifier.

Preliminary results of tests of 50 um thick UFSD pads and strip detectors with the proton beam of the CNAO hadron-therapy center of Pavia, Italy (proton fluxes up to 5*109 _Hz/cm2_{, FWHM 1 cm) will be presented.}

Waveforms collected from two aligned sensors have been analyzed to evaluate their counting and timing properties. Single beam particles are well separated and the fine time structure of the beam is resolved with nanosecond resolution. The detectors have been characterized in terms of time resolution (35 ps for single crossing), counting linearity, pile-up probability, signal degradation with the accumulated radiation dose. The experience gained from the test of sensors in the difficult environment of a therapeutic beam will be shared.