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%{\bf {Draft version III}}\\
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\begin{center}
\begin{large}
{\bf {\Large Fermilab Test Beam Proposal for Diamond Tracking Detectors}}\\
\end{large}
\vspace*{1.0cm}

J.~Conway$^1$, T.~Devlin$^1$, K.K.~Gan$^2$, H.~Kagan$^{2}$, R.~Kass$^2$,
M.~Mishina$^3$,
R.~Plano$^1$\\
S.~Schnetzer$^1$, S.V.~Somalwar$^1$, R.~Stone$^{1*}$,
G.~Thomson$^1$, W.~Trischuk$^4$, M.~Zoeller$^2$\\
\vspace*{0.15cm}
{\it {Rutgers$^1$, Ohio State$^2$, Fermilab$^4$,  Toronto$^4$ }}\\

\end{center}

%\vspace*{.3in}
\noindent
%$^{\dag}$Project Director\\
$^*$ Spokesperson



\vspace*{1.0cm}

This proposal requests beam time at Fermilab during the 1999
Fixed-Target Run to test Chemical Vapor Deposition (CVD) diamond
microstrip and pixel detectors.  Our research in this area began
nearly a decade ago when we realized that CVD diamond is a promising,
radiation-hard alternative to silicon.  Our work has been funded by
DOE, TNRLC, and NSF; we are currently funded by an NSF MRI grant.
Below we detail what we expect to accomplish in beam tests of diamond
detectors as well as what resources are required.

\subsection*{Introduction to Diamond Detectors}
 

In Table~\ref{tab:dparm}, we summarize the 
properties of diamond and, for comparison, those of silicon.
The most distinctive feature of diamond is its large band 
gap, 5.5 eV. This large band gap
along with the associated large cohesive
energy are responsible for much of the radiation hardness of diamond.
The large band gap also makes diamond an excellent electrical insulator. 
As a result, a large 
electric field can be applied without 
producing significant leakage current. Thus, there is no need for a 
reverse biased $pn$-junction and
the diamond detector functions much like a ``solid-state'' ionization 
chamber. 
Diamond has two additional properties that are favorable compared to silicon. 
Its smaller dielectric constant yields a smaller 
detector capacitance and, thereby, better noise performance of
the associated front-end electronics.  In addition, even though diamond 
is an electrical 
insulator, it is an excellent 
thermal conductor with a thermal conductivity exceeding  
that of copper by a factor of five.  A common 
problem with large strip detector systems is the management of the 
thermal load generated by the large number of electronic channels used 
in the detector readout. The handling of this thermal load would be
simplified if the detectors are constructed from diamond. 

Diamond appears ideal in many respects but it does have a
limitation: the large band gap which produces many of its outstanding
properties also means that its signal size is at most  
approximately half that of silicon for a given detector thickness in 
radiation lengths.  This may be compensated by lower front-end
electronic noise due to diamond's nearly non-existent leakage current 
and, for strip detectors, diamond's lower capacitive load.

\begin{table}[t]
\begin{footnotesize}
\begin{center}
\begin{tabular}{|lcc|} \hline
Property                                   & Diamond   & Si \\ 
\hline 
Band Gap [eV]                              & 5.5       & 1.12  \\  
Breakdown field [V/cm]                 & 10$^7$    & 3$\times10^5$ \\
Resistivity [$\Omega$-cm]                  & $>10^{11}$& 2.3$\times10^5$ \\
Intrinsic Carrier Density [cm$^{-3}$]      & $<10^{3}$ & 1.5$\times10^{10}$ \\
Electron Mobility [cm$^2$V$^{-1}$s$^{-1}$] & 1800      & 1350 \\ 
Hole Mobility [cm$^2$V$^{-1}$s$^{-1}$]     & 1200      & 480  \\ 
Saturation Velocity [km/s]                 & 220       & 82   \\ 
\hline
Mass Density [g cm$^{-3}$]                & 3.5       & 2.33  \\ 
Atomic Charge                              & 6         & 14 \\
Dielectric Constant                        & 5.7       & 11.9 \\ 
Thermal Expansion Coefficient [K$^{-1}$]   & 0.8$\times10^{-6}$  
                                                       & 2.6$\times10^{-6}$ \\
Thermal Conductivity [W m$^{-1}$ K$^{-1}$] & 1000-2000 &150 \\ 
Cohesive Energy [eV/atom]                  & 7.37      & 4.63 \\
\hline 
Energy to create e-h pair [eV]             & 13        & 3.6 \\ 
Radiation Length [cm]                      & 12.0      & 9.4 \\
Spec. Ionization Loss [MeV/cm]             & 4.69      & 3.21 \\
Ave. Signal Created/100 $\mu$m [e]         & 3600      & 8900 \\ 
Ave. Signal Created/0.1\% X$_0$ [e]        & 4500      & 8400 \\ 
\hline
\end{tabular}
\end{center}
\end{footnotesize}
\caption[tab:dparm]{\small
The physical properties of diamond and silicon at 293K.~\cite{zhao91}}
\label{tab:dparm}
\end{table}

In Fig.~\ref{diamond_det}, we show the basic principle behind the use
of diamond as a charged particle detector.  Several
hundred volts ($\sim$ 1 V/$\mu$m) is applied across a layer of diamond
a few hundred microns thick. When a charged particle traverses the
diamond, atoms in the crystal lattice sites are ionized, promoting
electrons into the conduction band and leaving holes in the valence
band. On average, 3,600 electron-hole pairs are created per 100~$\mu$m
of diamond traversed by a minimum ionizing track.
 These charges drift across the diamond in
response to the applied electric field producing a signal that can be
measured.

\begin{figure}[ht]
\begin{center}
  \parbox{3.5in}{\epsfxsize=\hsize\epsffile{diamond_det.eps}}
  \caption{A schematic view of a diamond detector.}
  \label{diamond_det}
\end{center}
\end{figure}
%\begin{figure}[ht]\centering
%  \epsfig{file=diamond_det.eps, width=3.5in}
%  \caption{A schematic view of a diamond detector.}
%  \label{diamond_det}
%\end{figure}

An interesting feature of diamond sensors is that they improve with
exposure to radiation for exposures up to about 1 kRad. 
This ``pump-up'' effect is due to an increase
of carrier lifetime caused by passivation of deep traps.
Because diamond has such a large band gap there may exist traps
more than 1 eV from the valence or conduction bands. Exposure to
radiation fills these traps with electrons (holes) produced by the radiation.
If the traps are far enough from the conduction (valence) bands, the
rate of thermal ionization of these traps will be slow and they will remain 
passivated for long times. We have found that diamonds kept in the 
dark remained pumped for at least three months. Exposure 
to light of the energy of the traps rapidly ionizes the traps and depumps the diamond. 

\subsubsection*{History}

 The collaborating institutions on this proposal bring with them an
extensive and unique set of capabilities in diamond detector research.
The groups from Rutgers and Ohio State pioneered the work on CVD
diamond detectors in the DIAMAS proposal~\cite{ssc_eoi} to the SSC in
1989.  Since 1994, these two groups have continued to play major roles
in diamond detector research as founders and leaders of the RD42
collaboration~\cite{rd42drdc94} at CERN.  From 1989 to 1993, the
DIAMAS collaboration pioneered the field of CVD diamond
detectors. This collaboration was the first to observe single minimum
ionizing particles in CVD diamond~\cite{d_nim1}.  In 1993, the first
calorimeter-quality diamond wafers were produced and a
diamond-tungsten calorimeter was constructed along with its readout
electronics~\cite{calorimeter_paper}.  This device achieved the same
energy resolution as a similar silicon-tungsten calorimeter
demonstrating that diamond calorimetry works.  This was also the first
demonstration of large-scale production of CVD diamond detector
material with 200 cm$^2$ of diamond produced in six months.
Figure~\ref{history} show the enormous progress we have made in
increased signal size over the past eight years.

\begin{figure}[ht]
\begin{center}
  \parbox{4.5in}{\epsfxsize=\hsize\epsffile{history.eps}}
  \caption{Pulse height of diamond detector for minimum ionizing 
track versus date.}
  \label{history}
\end{center}
\end{figure}



\subsubsection*{Results of Prior Testbeams}

Since 1994, the RD42 collaboration has worked on continuing to improve
the quality of CVD diamond material, constructing and testing diamond
microstrip and pixels detectors and testing the radiation hardness of
diamond detectors.  The first diamond microstrip detector was
constructed and tested in 1994 ~\cite{tracker_paper}. Since then,
diamond trackers constructed with 50~$\mu$m strip-pitch have yielded
12~$\mu$m spatial resolution, 98\% efficiency and 50:1 peak
signal-to-noise ratio with 1.5~$\mu$s shaping-time
electronics~\cite{rd42ldrb95,rd42ldrb95A,rd42lhcc97}.  In 1996, the
first diamond pixel detector was constructed attaining a 22:1 peak
signal-to-noise ratio. In this past year the first diamond tracker
(50~$\mu$m pitch) was operated with 25~ns peaking-time electronics
attaining a most probable signal-to-noise ratio of 7:1.


%Figure~\ref{d39_ph_res} shows the pulse height distribution and
%position resolution obtained with one of the better diamond trackers.
%This sample had an as-grown thickness of 1,600 $\mu$m. Material was then
%lapped from the substrate side until a final thickness of 675 $\mu$m
%was attained. 
%This procedure improved the signal size of the diamond by 40\%.
%The 675 $\mu$m thick sample was then 
%metallized as a 50-$\mu$m-pitch
%microstrip tracker and tested in a high energy pion beam. 
%The measured most-probable 
%charge (based on the sum of the pulse heights in the three strips nearest
%the track) is 4,200 electrons. A spatial resolution of 16~$\mu$m is 
%obtained by 
%comparing the hit
%cluster position recorded on the diamond with the extrapolated track from 
%silicon reference planes. 
%
%
%\begin{figure}[ht]\centering
%  \parbox{3.2in}{\epsfig{file=d39_117_charge.ps,width=3.2in}}
%  \parbox{3.2in}{\epsfig{file=d39_117_res.ps, width=3.2in}}
%  \caption{Pulse height distribution and position resolution
%           measured with a 100 GeV pion beam.} 
%  \label{d39_ph_res}
%\end{figure}

Figure~\ref{uts_sig} shows the pulse height distribution of a \mbox{432 $\mu$m} thick 
diamond microstrip device readout with VA-2 electronics (1.5
$\mu$sec shaping time). The pulse height
consists of the three-strip sum of the signal on the strip corresponding 
to the extrapolated hit plus the signal on each of the two neighboring strips.
The mean collected charge for $100~$GeV incident pions was 8,000 $e^-$
while the most probable signal corresponded to a collected charge of 
5,000 $e^-$. The ratio of the most probable signal size to single-channel
noise was 40 to 1.

\begin{figure}
\centerline{\epsfxsize 3in \epsffile[0 350 500 750]{/data/srs/steve/diamond_talk/uts5_826_ch3_fit.ps} }
\caption{Pulse height distribution of 432-$\mu$m thick detector with 
VA-2 electronics.}
  \label{uts_sig}
\end{figure}


Figure~\ref{uts_sig0} shows the distribution for signals less than
3,000 $e^-$ indicating a clear separation of the distribution from
zero. The spatial resolution achieved with this detector using a
center-of-gravity position algorithm is shown in
Figure~\ref{uts_res}. The residual distribution (measured hit position
- extrapolated hit position) has a standard deviation of 15 $\mu$m,
the digital resolution for 50 $\mu$m pitch.

\begin{figure}
\centerline{\epsfxsize 3in \epsffile[0 350 500 725]{/data/srs/steve/diamond_talk/uts5_826_ch3_zoom.ps} }
\caption{Detail of region near zero pulse height.}
  \label{uts_sig0}
\end{figure}

\begin{figure}
\centerline{\epsfxsize 3in \epsffile[0 350 500 725]{/data/srs/steve/diamond_talk/uts5_826_res.ps} }
\caption{Position resolution of 432-$\mu$m thick detector using a
  center-of-gravity position algorithm.}
  \label{uts_res}
\end{figure}


CVD diamond is inherently polycrystalline in 
nature. As a result, the level of spatial uniformity of the average signal 
size is of interest since it could effect the spatial resolution 
achievable through charge sharing. With current statistics, 
the uniformity can be probed at the 100 $\mu$m $\times$
100 $\mu$m level. Figure~\ref{uts_unif} shows the average signal size (grayscale coded)
of a 2 mm $\times$ 2 mm area of diamond in 100 $\mu$m $\times$
100 $\mu$m bins. On average, there are 40 tracks per bin. Comparing this
same plot with Figure~\ref{uts_scram} in which the hit position is randomly scrambled 
indicates a nonuniformity in the diamond. A 
quantitative measure of this nonuniformity is given by the ratio of the 
standard deviation of the average pulse height per bin to the average pulse
height. Finite statistics contributes about 8\% to this 
value. When this is subtracted in quadrature the residual nonuniformity
is about 30\%. For comparison, the width of the Landau distribution 
is about 23\% of its mean.

In the coming year,  
sufficient statistics needs to be accumulated to study the spatial uniformity 
at the 25 $\mu$m $\times$ 25 $\mu$m level. 
Similar effects can be seen when looking at the spatial dependence of
position resolution.  About the same level of statistics is needed 
to study resolution uniformity carefully and the same data set can be used as for the
spatial uniformity study. 



\begin{figure}
\centerline{\epsfxsize 3in \epsffile[0 0 550 705]{/data/srs/steve/diamond_talk/uts5_826_ph3_unif_100.ps}}
\caption{Mean collected signal charge vs. track hit position in 100 $\mu$m $\times$
100 $\mu$m bins.}
   \label{uts_unif}
\end{figure}


\begin{figure}
\centerline{\epsfxsize 3in \epsffile[0 0 550 705]{/data/srs/steve/diamond_talk/uts5_826_ph3_unif_100_scr.ps} }
\caption{Same as Figure~\ref{uts_unif} but with the track position randomly scrambled.}
   \label{uts_scram}
\end{figure}

A program has begun to test diamond pixel
devices bump-bonded to CMS and ATLAS pixel electronics. A successful test 
of a diamond device metallized with Ti/W
electrodes in a $50~\mu$m $\times$ 536 $\mu$m bricked pattern
shown in Figure~\ref{pix_pat} has been performed. The size of the device is 
4 mm $\times$ 8 mm and consists of 12 $\times$ 64 pixels. An ATLAS/3
readout chip was indium bump-bonded to this diamond by Boeing.

\begin{figure}
\centerline{\epsfxsize 3in \epsffile{/data/srs/steve/diamond_talk/pixel_atlas_lbl.eps} }
\caption{ATLAS/3 pixel pattern metallized on diamond.}
    \label{pix_pat}
\end{figure}

The device was tested in a 100 GeV/c pion beam at CERN.
Figure~\ref{pix_hits} indicates the hit occupancy. There are
several features to be noted. Two of the columns, 4 and 6, are ``hot''
due to a problem with the readout chip. 
These two columns were excluded from the analysis.
The left half of the device has a higher hit density than the right half
due to the beam not being centered. Figure~\ref{pix_dist} shows the distribution of
the number of hits per pixel for the left side of the detector
which saw the most beam.  Out of these 256 pixels, only 6 had no hits 
indicating that 98\% of the channels were successfully bonded.

\begin{figure}
\centerline{\epsfxsize 3in \epsffile[0 140 550 700]{/data/srs/steve/diamond_talk/pixel_atlas_hitmap.eps}}
\caption{Number of hits per pixel.}
    \label{pix_hits}
\end{figure}

\begin{figure}
\centerline{\epsfxsize 3in \epsffile{/data/srs/steve/diamond_talk/pixel_atlas_hits.eps}}
\caption{Distribution of number of hits per pixel.}
   \label{pix_dist}
\end{figure}

Of the 49,000 tracks that were extrapolated to have passed through
the active diamond area (excluding the regions under the two ``hot''
columns), there were 12,000 pixel hits that exceeded the 3,500 $e^-$
threshold of the electronics. Figure~\ref{pix_pul} shows the pulse height distribution
of these hits. Since the version 
of the ATLAS/3 electronics used was not radiation hard, the diamond bonded
to it was not able to be  
pumped up by exposing it to 1 kRad of ionizing radiation. 
Since this diamond had previously been configured as a microstrip tracker and
had been tested in a beam, its single strip pulse
height distribution when pumped was known. Overlaying this 
distribution 
onto the one measured for the pixel device, an efficiency
of 50\% would have been obtained if the diamond had been ``pumped''. 
Furthermore, the ATLAS electronics
will eventually be operated with a threshold of 2,000 $e^-$.
Based on the known charge distribution, 
the efficiency of the ``pumped'' diamond 
with a 2,000 $e^-$ threshold would be 85\%. 

\begin{figure}
\centerline{\epsfxsize 3in \epsffile{/data/srs/steve/diamond_talk/pixel_atlas_charge.eps}}
\caption{Collected charge in diamond pixel detector.}
   \label{pix_pul}
\end{figure}

The position resolution of the the diamond pixel is shown in Figures~\ref{pix_res_50}
and \ref{pix_res_530}. The resolution in the 50 $\mu$m direction is 14.8 $\mu$m and
in the 530 $\mu$m direction it is 140 $\mu$m. Both are approximately the 
digital resolution. 

\begin{figure}
\centerline{\epsfxsize 3in \epsffile[0 100 600 700]{/data/srs/steve/diamond_talk/pixel_atlas_res.eps}}
\caption{Pixel spatial resolution in 50 $\mu$m direction.}
  \label{pix_res_50}
\end{figure}

\begin{figure}
\centerline{\epsfxsize 3in \epsffile[0 100 600 700]{/data/srs/steve/diamond_talk/pixel_atlas_res_long.eps}}
\caption{Pixel spatial resolution in 530 $\mu$m direction.}
  \label{pix_res_530}
\end{figure}


\subsection*{Why Test Beams at FNAL Are Needed}

It is clear from the above discussion that CVD diamond material is
nearly of sufficient quality to meet criteria for use as vertex
detectors in future collider experiments.  Bench measurements using $^{90}$Sr
beta source are useful for coarse evaluation of newly produced
material and for feedback to manufacturers.  But a beam test
environment is clearly the only way to get detailed information on the
potential tracking performance of the material at hand in a relatively
short amount of time.

The following list summarizes what needs to be measured for both strip
and pixel detectors:

\begin{itemize}

        \item spatial resolution
        \item hit efficiency
        \item comparison of pulse height distribution with that from
                $^{90}$Sr measurements
        \item  pulse height uniformity studies (high statistics needed)
        \item  resolution uniformity studies (high statistics needed)
        \item  Lorentz angle determination within a magnetic field

\end{itemize}

During the last few years there has been only one major supplier of
detector grade CVD diamond.  It takes several weeks to grow a wafer
and comparable amounts of time to prepare the surface, deposit
electrodes, examine test pieces from the wafer, and make final sample
preparations for a test beam.  Based on this growth-to-testbeam cycle
time our experience has been that three to four test beam studies per
year have been sufficient to keep up with the production of new
diamond.  However, during 1999 we expect to have material from two new
manufacturers to test.  We cannot expect any increased amount of
testbeam time at CERN.  We therefore need to look for alternative
locations such as FNAL.

There are numerous advantages to incorporating a test beam at FNAL
into our overall R\&D program.  Besides the obvious savings in travel
time and expense, we have much closer connections to ongoing
experiments at FNAL (CDF, KTeV, CMS, et al.) and hope to exploit these
to share resources.  Conversely, we are soon to lose two of our close
(and hardworking) contacts at CERN due to graduation.  Faster
turnaround of data analysis can be expected from an FNAL testbeam
because of existing high-speed networks in place between the Lab and
our North American universities.  This will provide more rapid
feedback to the diamond manufacturers as well as to the overall R\&D
effort.

We plan to test specific prototype detectors, equivalent to those to
be used in CMS and Atlas, CDF and perhaps BTeV.

\subsection*{Proposed Test Beam Program and Requirements}

We have found that about  five days of dedicated beam time is 
optimal to test five to ten devices.  We are DAQ deadtime limited above
100 Hz into a 5 cm$^2$ area.
High statistics are needed for
some devices to determine position resolution and uniformity.  Others
need to be studied more quickly with, for example, frequent bias
adjustments.  
During a given 5 day test beam, we plan to test at least three 1
cm$^2$ strip detectors, one larger strip detector 2 cm x 4 cm, and at
least two pixel sensors bonded to pixel electronics.


We will supply the overall tracking telescope, readout electronics,
and DAQ.  Resources needed at FNAL are:

   \begin{itemize}

        \item charged particle beam with
        energy $>$ 10 GeV to minimize effects due to multiple scattering

        \item flux of $>$ 100 / cm$^2$ / sec into an area of 5 cm$^2$

        \item minimal space in test beam hall: a ~1 m$^2$ table 
                just below the beamline

        \item a BM 109 for Lorentz angle drift studies

        \item in the beam we need to locate a scintillator trigger, 8
                planes of silicon strip detectors 
                for precision tracking and
                several planes of diamond detectors under test
                
        \item the above items represent a total of about 6\% $X_o$

        \item No significant
                electronics space is needed near the beamline.  Some
                rack and table space is needed in a nearby counting
                room

        \item Cabling to the counting room is also quite
                minimal

   \end{itemize}

Due to the expected increase in new diamond material, we are
requesting 3 runs spread out over the 1999 Fixed-Target running period
consisting of 5 days of beam time/run with occasional beamline access
every 8 hours.  We will be ready for beam in May of 1999.


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