Speaker
Description
Members of the Timepix family of detectors [1], [2] have been successfully used
as single-layer particle identification devices in space dosimetry. The advantage
of this approach over standard absorbed dose measurements is that it enables a
more accurate estimate of biological dose because in addition to absorbed dose,
the composition of the radiation field can be analyzed simultaneously. This is
made possible by the analysis of signal characteristics, i.e. the shape of socalled
clusters of multiple hit pixels that are formed under ion irradiation and
the charge measured in each pixel of the cluster.
A similar situation arises in hadrontherapy with heavy ions and the particle
identification and energy-deposition measurement capabilities of Timepix detectors
are of interest for hadrontherapy dosimetry and treatment monitoring.
The biological effects of radiation on tissues are closely linked to the linear energy
transfer (LET) value of the radiation. In hadrontherapy, the increased
biological effectiveness of protons and especially heavier ions such as He, C and
O is concentrated in the Bragg peak region. Treatment planning depends to
a large extent on accurate estimations of ions’ stopping power in the tissue to
correctly determine the local LET and consequently the local biological effect,
and also the range of the ions. In this context, the Timepix family is a promising
detector technology, since it could be used to measure LET spectra in different
depths along the Bragg curve. Furthermore, the stopping power distribution
along the beam direction,which translates to the ions’ range, could be measured
by means of ion-beam imaging right before the treatment. Here, high-energetic
ion beams that can traverse the patient at very low doses are used [3].
However, accurate measurement of the charge deposited by the heavy ions in
the Bragg peak remains a challenge, as the amount of charge deposited per pixel
often exceeds the linear response range of the pixel’s front electronics. Timepix
[1] and Timepix3 [2] in particular suffer from the so-called volcano effect, which
manifests itself in a corruption of the charge information in the center of the
pixel cluster.
A systematic investigation of the pixel response to high input charge (>>100ke)
is difficult due to the limited availability of ion pencil beams with a diameter
in the μm-range and low fluence rates. High intensity laser pulses provide a
well-controlled and scalable alternative mechanism to generate high LET charge
deposition events in the sensor. The aim of this work is to characterize the nonlinear
behavior of the Timepix3 pixel front end using high intensity laser pulses
and to derive corrections for the charge lost in the volcano. Cluster parameters
such as cluster size, total ToT and the hit of secondary pixels generated in the
Timepix3 nonlinear regime are analyzed and assessed with respect to their potential
benefit in volcano correction strategies.
References
[1] X. Llopart, R. Ballabriga, M. Campbell, L. Tlustos, and W.Wong. Timepix,
a 65k programmable pixel readout chip for arrival time, energy and/or photon
counting measurements. Nuclear Instruments and Methods in Physics
Research Section A: Accelerators, Spectrometers, Detectors and Associated
Equipment, 581(1):485–494, 2007. VCI 2007.
[2] T Poikela, J Plosila, T Westerlund, M Campbell, M De Gaspari, X Llopart,
V Gromov, R Kluit, M van Beuzekom, F Zappon, V Zivkovic, C Brezina,
K Desch, Y Fu, and A Kruth. Timepix3: a 65k channel hybrid pixel readout
chip with simultaneous toa/tot and sparse readout. Journal of Instrumentation, 9(05):C05013, may 2014.
[3] M Martišiková, T Gehrke, S Berke, G Aricò, and O Jäkel. Helium ion beam
imaging for image guided ion radiotherapy. Radiat Oncol., 13(1):109, june
2018.
