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\centerline{\huge The BESSY Fluorescence Chamber}
\section{Overview}
\begin{center}
\begin{tabular}{|llcc|}
\hline
\multicolumn{4}{|l|}{Spokespersons:}\\
\hline
 & & BESSY & Humboldt-Universit\"at\\
\hline
 & Tel.: & +49-(0)30-82004-198 & +49-(0)30-2093-7756\\
R\"udiger Mitdank & Fax: & +49-(0)30-82004-149 & +49-(0)30-2093-7729\\
 & E-Mail: & mitdank@exp.bessy.de & mitdank@physik.hu-berlin.de\\
\hline
 & Tel.: & +49-(0)30-82004-198 & +49-(0)30-2093-7767\\
Ralph M\"uller & Fax: & +49-(0)30-82004-149 & +49-(0)30-2093-7729\\
 & E-Mail: & rmueller@exp.bessy.de & ralph.mueller@physik.hu-berlin.de\\ 
\hline
 & Tel.: & +49-(0)30-82004-198 & +49-(0)30-2093-7744\\
Mathias Schneider & Fax: & +49-(0)30-82004-149 & +49-(0)30-2093-7729\\
 & E-Mail: & mschneider@exp.bessy.de & matthias.schneider@physik.hu-berlin.de\\ 
\hline
\end{tabular}
\end{center}

\subsection*{Typical Experiments}
X-ray absorption experiments (EXAFS, NEXAFS etc.)

\subsection*{General Informations}
The fluorescence chamber is a comparatively simple unit for x-ray absorption
experiments on solids, surfaces and interfaces with a semiconductor x-ray
detector. The ultra-high vacuum chamber is evacuated by a turbo-molecular
pump ($240~l/s$) and  a titanium sublimation pump. The base pressure is
typically $5\cdot10^{-9}~mbar$.

\subsection*{X-ray Detector}
A high purity germanium detector is fixed 90$^\circ$  with respect to the
incident synchrotron radiation. Depending on the space at the used beamline, it
also can be used fixed with an angle of 45$^\circ$. The working distance between sample and
detector can be varied from $5~mm$ to $140~mm$. The best energy resolution is
$85~eV$ at $290~eV$ (C 1s) and $110~eV$ at $1740~eV$ (Si 1s). In particular,
the detector is suited for studies of bulk materials and buried layers.

\subsection*{I\boldmath$_0$ Detector}
A gold grid (80\% transmission) controls the incident synchrotron radiation
by the generated photo electrons. I$_0$ is used for the normalization of 
the measured absorption data.

\subsection*{Electron Yield Detector}
With the electron yield detector total yield measurements in x-ray 
absorption experiments are possible. It is fixed 45$^\circ$ with respect to the incident
synchrotron radiation. The working distance is $100~mm$.

\subsection*{Manipulator}
The manipulator allows movements in the following regions: x, y $\pm15~mm$; 
z: $300~mm$; $\Theta$: 0...360$^\circ$. As well for room temperature as for cooled
measurements (liquid nitrogen) there is a sample port where samples can be placed
under UHV conditions using a sample transfer mechanism (see below).

\subsection*{Sample Preparation}
If required, a sputter gun ($5..10~kV$ Ar-ions) can be used inside the 
chamber. Otherwise the samples can be prepared in the BESSY chemical lab.

\subsection*{Computer Control}
\begin{tabbing}
Hardware:\hspace{5mm}\=IBM-AT compatible computer (386)\\
 \>PCA-II interface card for the germanium detector\\
Software:\>PCA-II program for pulse heights analysis
\end{tabbing}

\subsection*{General Notes about Working at Experimental Stations}
You should keep the experimental station (measurement chamber, beam line
and equipment) clean and in order - also in your own interest. In addition,
you should remember everytime the safety regulations (especially 
those about radiation safety and vacuum).

\section{Pumping Down and Venting the Fluorescence Chamber}

\subsection*{Pumping Down}
\begin{enumerate}
\item Connect all vacuum parts:
\begin{figure}[h]
\begin{center}
\includegraphics[width=0.85\textwidth]{vakanl_e.eps}
\end{center}
\end{figure}
\item Connect media (compressed air, cooling water, exhaust gas line, power).
\item Close valves BV, V1 and the valve to the air-lock. Open valve V2.
\item Switch on pumping unit (forepump VP and turbo pump TP) with the right button on the controller.
\item Open manually operated valve V1. Switch on measuring instrument M201.
\item The turbo pump will reach step 2 after $t\approx5~min$ and step 6 after 
	$t\approx13~min$.
\item Turn on UHV measuring gauge TPG300 below $p<2\times10^{-5}~mbar$.
\end{enumerate}

\subsection*{Venting}
\begin{enumerate}
\item Close valve to the beam line and to the air-lock.
\item Move Ge detector totally out of the experimental chamber. Close
manually operated valve V3.
\item Disconnect total yield detector (battery box, NIM chassis).
\item Turn off I$_0$ measuring instrument.
\item Venting with liquid nitrogen through the venting valve (BV):
\begin{itemize}
\item Open BV slowly until $p_{TPG300} = 10^{-4}~mbar$.
\item Close manually operated valve V1.
\item Switch off pumping unit.
\item Turn off UHV measuring instrument TPG300.
\item Open BV more. If TP makes too much noise while speeding down close BV
     a little bit.
\item Open BV totally after the turbo pump has come to a standstill.
\item Turn off forepressure measuring instrument M201.
\end{itemize}
\item The chamber is vented when the manipulator bellow has become loose.
\end{enumerate}

\section{Using the Air-Lock}

The air-lock is generally attached to the fluorescence chamber. It is 
connected through th manually operated valve V4 with the fluorescence chamber.
The following sketch shows the setup. \\

\begin{center}
\includegraphics[width=0.85\textwidth]{schleuse.eps}
\end{center}

\subsection*{Venting the Air-Lock}
\begin{enumerate}
\item The valve to the fluorescence chamber must be closed.
\item Liquid nitrogen is used for venting.
\item Switvh off UHV measuring gauge (TPG300).
\item The automatic venting valve at the turbo pump is used for venting.
\item Switch off the pumping unit TSU-065-D. The air-lock vents automatically.
\end{enumerate}

Now the air-lock can be opened at the z-translator. Then you can take off the
rotary drive with the sample storage. The samples can be removed and replaced.\\

\subsection*{Pumping Down the Air-Lock}
\begin{enumerate}
\item Switch on pumping unit TSU-065-D. The turbo pump will reach normal
	oparation after 3 to 5 min.
\item Switch on UHV measuring gauge (TPG300).
\item The valve V4 can be opened if the difference between the pressure in
	the fluorescence chamber and the pressure in the air-lock is a factor 
	of 10. The pressure in the fluorescence chamber will then double after 
	opening the valve V4.
\end{enumerate}

\subsection*{Transfering Samples}

The components of the transfer system are shown in the following sketch. To
transfer a sample, thread the transfer fork heads into the bore holes of the
\begin{figure}[ht]
\begin{center}
\includegraphics[width=0.43\textwidth]{transf.eps}
\end{center}
\end{figure}
wedge of the sample holder. Push the transfer head - maybe with light turning
of it - as far as the screw of the sample holder is in the locking position of
the transfer head. Then unscrew the sample holder from the sample port by turning
counterclockwise. The screw of the sample holder must be locked! Then the sample
holder can be pulled out of the sample port.\\

For screwing the sample holder into the sample port push it straight into th
port. By turning it clockwise it will be screwed into the port and be unlocked.
Then the transfer head can be pulled back.\\

{\bfseries 
\underline{IMPORTANT:}
\newline
Do not transfer if the sample holder is not locked in the transfer head.
\newline
Do not close the manually operated valve V4 (between air-lock and fluorescence
chamber) if the transfer rod has not pulled back completely.
\newline
Do not screw the sample with force.}

\section{The Germanium Fluorescence Detector}

\underline{Normal pressure:}\newline
$5\cdot10^{-9}~mbar$ in the detector chamber ($=2~\mu A$ at the small
gettering pump)

\underline{Detector cooling:}\newline
Use of liquid nitrogen ($7~l$ dewar). {\bf One filling will be enough for six days,
that means you should refill it twice a week}. Without cooling the small gettering
pump can't maintain the vacuum. A half-full dewar leads to a N$_2$ alarm 
(accoustic as well as visual) at the TX1236 module if the detector 
electronics are turned on.

\subsection*{Moving into Measuring Postition}

\begin{enumerate}
\item Close valve to beam line.
\item Turn the handwheel of the valve between detector and experimental
	chamber V3 until the red pin juts out completely ($\approx 5~mm$) 
	from the handwheel.
\item Move the detector by constant and counterclockwise rotation of the 
	handle into the experimental chamber.
\item Mark the position of the smallest distance between detector and 
	sample manipulator (observed through a viewport). The bellow can
	be compressed down to $8~cm$.
\item Darken the experimental chamber, turn off all light sources 
	(filaments of measuring gauges, titanium sublimation pump, ...).
\item Turn on the detector electronics (NIM chassis).
\end{enumerate}

\subsection*{Finishing the Measurement}

\begin{enumerate}
\item Close valve to beam line.
\item Turn off the detector electronics (NIM chassis).
\item Move the detector out of the chamber constantly by clockwise rotation 
	of the handle (arrow direction $\rightarrow$) to the stop.
\item Close the valve between detector and experimental chamber by a constant
	rotation of the handwheel until the red pin disappears totally in the
	handwheel and you feel a resistance in the rotation.
\end{enumerate}

\subsection*{Detector Electronics}

If light (zero order, filaments of measuring gauges, titanium sublimation 
pump, lighting of the sample area for adjustments, ...) falls into the 
experimental chamber the detector electronics must be turned off!
\begin{enumerate}
\item Turn on the PC. Start program \fbox{\tt pca.bat} or \fbox{\tt pcaklm.bat}.
\item \underline{Electronic connections}\\
	Detector cable with pulse processorr TX1255 (in NIM chassis)\\
	Pulse processor with voltage module TX1236 (in NIM chassis)\\
	Pulse processor {\sf FD} with rate meter {\sf INPUT} (in NIM chassis)\\
	Pulse processor {\sf INPUT} with TC245 {\sf Signal-IN} (in NIM chassis)\\
	Pulse processor {\sf OUTPUT} or TC245 {\sf UNI-OUT} with PCA interface 
	card {\sf INPUT} (in PC, right).
\item Turn on NIM chassis.
\item After $30~min$ the initialization of the electronic should be finished
	and the detector can be used for measurements.\\
	Dark count rates at the TX1255:
\begin{itemize}
\item high count rate: 100 cps ($\rightarrow$ rate meter), correction with
	fast disk.
\item low count rate: 200 cps, correction with slow disk.
\end{itemize}
\end{enumerate}

\subsection*{Standard Amplifier Adjustments}

\begin{tabular}{ll}
Pulse processor TX1255: & Gain 4\\
Tennelec TC245: & Gain grob 500, fein 13\\
	 & PUR - OFF\\
	 & PA-RESET
\end{tabular}\\
\underline{Ranges of linear amplification}\\
(Measure the total count rate at the rate meter!)
\begin{itemize}
\item Pulse processor TX1255 \newline
	(no shifts of the energy calibration)\\
	\begin{tabular}{l@{:}cr}
	low count rate & \hspace{5mm} & up to 2500 cps\\
	medium count rate & & up to 5000 cps\\
	high count rate & & up to 7000 cps
	\end{tabular}
\item TC 245 (peaking time)\newline
	(shifts of the energy calibration coresponding to peaking time.)\\
	\begin{tabular}{r@{:}cr}
	$24~\mu sec$ & \hspace{5mm} & up to 3000 cps\\
	$12~\mu sec$ & & up to 7000 cps\\
	$3~\mu sec$ & & up to 20000 cps
	\end{tabular}
\end{itemize}
The intensity can be varied by the distance between detector and sample.

\subsection*{PCA-II Interface Card}

The PCA interface card allows pulse heights analysis of the peaks formed by
the amplifiers, graphic representation, energy calibration, choice of an
interesting spectral range (setting a window) and delivery of TTL signals
at the outputs. It's possible to save pulse heights spectra either in ASCII
or binary.\\
\begin{itemize}
\item \underline{Standard adjustments:}\newline
	\begin{tabular}{lcl}
	MODE & \hspace{5mm} & PHA (pulse heights analysis)\\
	Memory Group & & E1 (x-axis: 1024 memory blocks)\\
	Conversion Gain & & 2048 (10~V INPUT = memory 2048)\\
	Display & & 1024 (x-axis: 1024 pixel)
	\end{tabular}
\item \underline{Setting a window and outputs:}
	\begin{enumerate}
	\item SCA
		\begin{itemize}
		\item Acceptance range of the interface card
		\item Adjustment with a rotatory resistor (U, L on the back side 
			of the interface card)
		\end{itemize}
	\item LTB
		\begin{itemize}
		\item Choose ROI (region of interest) by software
			({\sf F9, F10, Del, Alt-R})
		\end{itemize}
	\end{enumerate}
\item \underline{Important keys:}\\
	\begin{tabular}{ccl}
	{\sf F1} & \hspace{5mm} & Start Measurement (on) - Finish Measurement (off)\\
	{\sf F2} & & Disable erasing - enable erasing\\
	{\sf Strg-F2} & & Erase spectra\\
	{\sf Alt-..} & & Activate menu (.. = 1. letter)\\
	{$\leftarrow , \rightarrow$} & & Move cursor on x-axis (=energy).\\
	{$\uparrow , \downarrow$} & & Scale y-axis (linear .. logarithmic)\\
	right mouse button & & Turn from menu to cursor\\
	left mouse button & & Execute operation\\
	\multicolumn{3}{c}{Further information and help in the PCA-II manual}
	\end{tabular}
\end{itemize}

\subsection*{Detector De-icing}
Because of the permanent detector cooling a thin ice layer grows on the
detector. The ice layer causes a stronger absorption of the x-ray radiation.
While analysing energies above the oxygen K edge one will have additional
O-K$_{\alpha}$ radiation which will lead to further errors in intensity
(high O peak at $525~eV$, stronger absorption of other lines). Therefor
the detector should be de-iced before each measurement period. For this reason
a de-icing control unit is used which heats the head of the detector 
cyclically and regulatedly. The pressure increases for a short time up to
$10^{-5}~mbar$. This forbids to do this procedure inside the detector chamber.
\begin{enumerate}
\item Move the detector into the experimental chamber (The valve to the
	beamline must be closed).
\item Unplug the LN (liquid nitrogen) sensor from the pre-amp housing (LN 
	alarm at module TX1236 activated!).
\item Plug in the detector de-icer control box into the LN sensor jack 
	located on the pre-amp housing.
\item The green "COMPLETE" led should be lit. This indicates there is power
	to the control box.
\item Push the button on the detector de-icer control box front panel. The 
	"COMPLETE" led should go off and the red "DE-ICING" led should light.
\item The "DE-ICING" led will remain on for approximately 1 hour. When the
	"COMPLETE" led comes on the detector should be de-iced.
\item Unplug the detector de-icer control box from the pre-amp housing and 
	plug in the LN sensor.
\item The detector can now be used but it may take a while for the detector
	to completely settle down ($\approx 30 min$).
\end{enumerate}

\section{Total Yield Measurements}

Connections:
\begin{figure}[h]
\begin{center}
\includegraphics[width=0.85\textwidth]{chantr_e.eps}
\end{center}
\end{figure}
\end{document}
