12th Vienna Conference on Instrumentation - VCI 2010

Study of avalanche fluctuations and energy resolution with an InGrid-TimePix detector

16 Feb 2010, 14:50

25m

HS 1 (TU Vienna)

Contributed Talk Gaseous Detectors 3

Speaker

Paul Colas (CEA/Irfu)

Description

The energy and spatial resolutions of a Micromegas detector is fundamentally limited by gain fluctuations, among other sources of degradation of these resolutions. Also the precise shape of the gas gain distribution determines the efficiency for single electron detection. These two reasons motivate our study of avalanche size fluctuations, using a new tool which is a combination of an integrated Micromegas (InGrid) onto a 65,000 pixel CMOS chip (TimePix). Electrons produced by a radioactive source are separated by diffusion in the gas. Therefore they can be counted and the fluctuation of their number can be measured. Doing this at various gas gains allows the shape of the gain fluctuations, assumed to follow a Polya parametrization, to be constrained. Also, the linearity of the Time-Over-Threshold measurement with deposited energy above a threshold of about 5000 electrons is used to obtain a direct measurement of the upper end of the gas gain fluctuation distribution, using a set of single incoming electrons separated from others by diffusion. As a by-product of the first measurement, the average energy per ionization is measured in several gas mixtures and the Fano fluctuation factor is directly determined. The latter information, together with measurements of the energy resolution on the Iron 55 Kalpha line, is used to constrain again the Polya parameter.

Summary (Additional text describing your work. Can be pasted here or give an URL to a PDF document):

Authors: D. ATTIÉ1), M. CAMPBELL2 ), M. CHEFDEVILLE3) , P. COLAS1), E. DELAGNES1) , K. FUJII4) , Y.GIOMATARIS1) , H. VAN DER GRAAF4) , X. LLOPART2) , M. LUPBERGER1) , J. SCHMITZ6), J. TIMMERMANS4) , M. TITOV1)

1) CEA Saclay ; 2) CERN ; 3) LAPP Annecy ; 4) KEK ; 5) NIKHEF; 6) Twente

The 5.9 keV x-ray record resolution of 5% r.m.s. for a gaseous ionisation detector was obtained with an integrated micromesh on Silicon (InGrid) and with a Microbulk Micromegas detector. It is interpreted as a sum of contributions from primary ionization fluctuations (Fano fluctuations) and gain fluctuations of various origins. A Micromegas (Ingrid) integrated on a 65000 pixel chip (TimePix) was used to measure the number of primary electrons released by 5.9 and 3 keV x-rays in Argon mixtures, as well as its fluctuations. The single electrons are separated by diffusion on individual pixels and detected with efficiency close to 100%. This allows a first direct and precise measurement of the average energy per ionization in such a mixture and a direct measurement of Fano fluctuations. By subtracting quadratically the Fano fluctuations from the measured width of the Iron 55 peak, one obtains a measurement of the gas gain fluctuations, assuming the collection efficiency to be close to 100%.
Assuming the fluctuations of z=G/<G> to follow the Polya functional form:
P(z) = mm/Gamma(m) z(m-1) exp(-mz),
the m parameter is determined.
Two other determinations are made possible by the TimePix chip:
- The dependence of the number of observed electrons released by 3keV X-rays (escape line in Argon) with the gain allows the efficiency for single electron detection to be determined, which in turn constrains the theta parameter.
- Using the Time-over- Threshold capability of the chip, the high-gain end of the gain distribution is directly measured.
These three determinations are compared between them and with the present measurements available in the literature.

This work was supported in part by the European Communities under the 6th Framework Program “Structuring the European Research Area” under contract RII3-026126.

Presentation materials

Slides

2_Colas.pdf

2_Colas.pptx