\documentclass{elsart}

\usepackage{graphicx,rotate}
\DeclareGraphicsExtensions{.eps,.ps,.eps.gz,.ps.gz}

\begin{document}

\begin{frontmatter}

\title{Gaseous Pixels}
\author{E.N. Koffeman}
\address{Nikhef, Kruislaan 409, 1098 SJ Amsterdam, The Netherlands}

\begin{abstract}

Several years ago a revolutionary miniature TPC was developed using a pixel chip with a Micromegas 
foil spanned over it. 
To overcome the mechanical stability problems and improve the positioning accuracy while 
spanning a foil on top of a small readout chip a process has been developed in which a Micromegas 
like grid is applied on a CMOS  wafer in a post processing step. This aluminum grid is supported on 
insulating pillars that are created by etching, after the grid has been made. 
The energy resolution (measured on 
the absorption of the X-rays from a $^{55}$Fe source) was remarkably good.
Several geometries have since been tested and we now believe that a 'Gas On  Slimmed Silicon Pixel chip' (Gossip) 
may be realized. 
The drift region of such a gaseous pixel detector would be reduced to a millimeter.
Such a detector is potentially very radiation hard (SLHC vertexing) but ageing and sparking must be 
eliminated. 

\end{abstract}
\begin{keyword}
Pixels,
Ingrid,
Gossip.
\end{keyword}
\end{frontmatter}


\section{Principle of operation}

Gossip is a pixel detector without a solid state detection layer. Instead the signal is formed in a 
thin gas layer (1~mm) and amplified in a gap (50~$\mu$m) between the surface of a CMOS chip  and 
a Micromegas-like grid~\cite{art:micromegas}. A minimum ionizing particle will on average produce about ten ion-electron pairs 
in the 1 millimeter drift space. These electrons drift toward the grid through the low field drift 
region (100~V over 1~mm). The maximum drift time is only 16 ns due to the small 
size of the drift distance. Between the grid and the readout chip a high electric field is created in 
which each electron will initiate an avalanche. With a potential difference of 400 V over a 
distance of 50~$\mu$m, a gas multiplication or gas gain of 1000 can be reached. 
The basic principle is shown in figure~\ref{fig:basic}. 

\begin{figure*}
\begin{center}
\includegraphics[width=0.99\textwidth]{basic}
\caption{Principle of operation of a Gossip sensor.}
\label{fig:basic}
\end{center}
\end{figure*}

It has been shown that  the Medipix chip~\cite{art:medipix2} 
can be used to detect signals created by an electron 
induced avalanche when a Micromegas is mounted on the chip~\cite{art:mpixandgas}. For these measurements a drift volume 
of 14~mm was used thus constructing a small time projection chamber. At present we are developing 
a detector with a 1~mm drift region over a readout chip that carries an integrated grid. An existing chip
(like ATLAS or CMS pixel chip)  will be used to build the first true Gossip prototype.   
Until now several grid geometries have been tried and proved to be successful.
The integrated grid (InGrid~\cite{art:ingrid}) can be adapted to match any pixel pitch.

Recently the first measurements were performed with a drift region as small as 1.2~mm. In this measurement
only a single electrode was used in combination with a charge integrating preamplifier with a slow shaping time of a few $\mu$s (Amptek 250). A $^{90}$Sr source was used to generate minimum ionizing particles. 
In figure~\ref{fig:signal} the resulting pulse height distribution is shown. The line shows a fit based on the assumption that the number of primary electrons fluctuates according to a Poisson distribution. This distribution is 
convoluted with noise and gain fluctuations. The voltages on the cathode and the grid were -800V and -600V respectively. The voltage difference between the grid and the anode was 600V corresponding to a gain of 10$^3$. 

\begin{figure*}
\vspace{1cm}
\begin{center}
\includegraphics[width=0.49\textwidth]{signal}
\caption{Measured pulse height distribution in 1.2 mm drift region. The primary electrons create an avalanche
in the 50~$\mu$m between the Micromegas and the anode (in this setup a single electrode). The peak around zero is 
corresponds to a fraction of 4\% empty triggers caused by Bremsstrahlung.}
\label{fig:signal}
\end{center}
\end{figure*}


\section{ Radiation hardness}

Through constant flushing of the gas volume, direct radiation damage is usually not an  issue in gaseous 
detectors. Ageing processes  may cause however the formation of deposits on the electrodes. 
First ageing tests do not show a significant degradation in performance of the chamber.  
A setup with a Micromegas grid has been irradiated with X-rays during two months. The charge collected on the anode
was 0.3C/mm$^2$ which is equivalent to 2$\cdot$10$^{16}$ charged particles per cm$^2$.
The gain decreased with a factor of four but was fully recoverable by increasing the field.
In the proposed layout the gas gain and the electrical field near the anode are very  moderate 
compared to most wire chambers.  

\section{Spark protection}

In principle each single electron created in the drift region (on expects roughly 10 electrons 
from  a minimum  ionizing particle)  will initiate the development of an avalanche in the region 
between the grid an the anode. When a given particle deposits 
much more energy and thereby releases many primary electrons, this may cause such a huge avalanche 
that the readout chip is actually destroyed. This could be due to melting or evaporation of the surface layers or
to an over-voltage on the input of the CMOS preamplifier. 
Without protection these effects are indeed observed in the lab.  

Two protection mechanisms are proposed and under construction. Firstly an amorphous silicon layer is applied 
on top the CMOS wafer before the grid is applied. This highly resistive layer is supposed to spread the charge
caused by a spark. The layer also offers direct protection against melting of the thin surface layers on 
the input pads. Secondly, wafers are being equipped with double grids on top 
of each other (TwinGrid). The TwinGrid will be operated such that 
the avalanche or gas gain takes place between the first and second grid. The region between the second grid
and the chip will have a lower field and serves to transport the signal to the chip without further amplification.
This structure actually has some other interesting consequences since the second grid may suppress the ion signal. 
As a result the signal development may become much faster (electron drift over 50~$\mu$m).


In figure~\ref{fig:twingrid} a picture of the TwinGrid is shown.
The insulating pillars are easily recognized as well as the double layer of grids. 
The pitch between two holes in the grid is 50~$\mu$m.

\begin{figure*}
%\begin{figure*}[hptb]
\begin{center}
\includegraphics[width=0.99\textwidth]{twingrid}
\caption{TwinGrid test structure. Two grids on top of each other to prevent sparks to the CMOS chip.}
\label{fig:twingrid}
\end{center}
\end{figure*}



\section{Readout chip}

In 2005 a preamplifier circuit was designed  that exploits the fact that the input capacitance of a Gossip detector is very low compared to present detectors. The first prototype of the fast (40 ns peaking time), low-noise (ENC = 60 e- (RMS)  and low-power (2 $\mu$W per channel) input circuit for the Gossip chip has been successfully implemented in 0.13 CMOS technology. At the end of 
2006 the design of the first small scale pixel chip with the same preamplifier but know equipped with a TDC for each pixel 
has been submitted.

\section{ Outlook}

The material in the Gossip detector will be dominated by the CMOS chip.
Since no bump bonding is required it should be possible to (back)thin the readout chip. 
Sofar no wafer thinning has been tried. 
The absence of bump bonding in the production process may give substantial saving in the costs. 

To fill the detector with gas we propose to fully surround it with gas at ambient pressure so the
construction does not need to accommodate pressure differences. 
Since there is no radiation damage of the gas there is no need (no harm either) in cold operation of the detector. 
A modest cooling of the front-end electronics will be necessary but the entire system can run at room temperature.

A prototype Gossip detector is being developed. This detector combines for the first time all elements of the Gossip principle. 
The CMS pixel chip will be used with a post processed grid. 
Several detectors will be made and put together in a beam telescope.
At the same time developments will continue in the design of a front-end chip that will fully profit from the negligible input capacitance and   reduced power consumption. 
 
\begin{thebibliography}{99}
%
%
\bibitem{art:micromegas}
Y. Giomataris et al., Nucl. Instr. and Methods A, {\bf 376}, 1996, p29

\bibitem{art:medipix2}
X. Llopart et al., IEEE Trans. Nucl. Sci., {\bf NS-49}, 2002, p2279

\bibitem{art:mpixandgas}
M. Campbell et al., Nucl. Instr. and Methods A, {\bf 540}, 2005, 295 - 304

\bibitem{art:ingrid} 
M. Chefdeville et al., Nucl Instr and Methods A, {\bf 150}, 2006,  200-203

\end{thebibliography}
\end{document}
