Scintillation efficiency of nuclear recoils in a CaF$_{2}$(Eu) crystal %for dark matter search
R. Hazama\thanksref{jsps}
%The energy dependence of t The scintillation efficiency (quenching factor) of fluorine and calcium nuclei recoiling in a CaF$_{2}$(Eu) crystal %CaF$_{2}$(Eu) crystal for calcium and fluorine nuclei was measured in an energy region relevant to dark matter searches. %in 25 $\sim$ 91 keV and 53 $\sim$ 192 keV recoil energy, respectively. The recoiling nuclei were obtained via the $^{19}$F(n,n') and $^{40}$Ca(n,n') reactions, where the neutron beam was provided via the d(d,n)$^3$He reaction. %, which gives appropriate low energy recoils. %the $^{19}$F(n,n') and $^{40}$Ca(n,n') reaction. The quenching factor of F and Ca nuclei in the CaF$_2$(Eu) crystal was found to be 11 $\sim 20 \%$, and 9 $\sim 23 \%$ relative to the electron equivalent energy for 53 $\sim$ 192 keV and 25 $\sim$ 91 keV recoil energies, respectively, %for the F and Ca nuclei, respectively %and found to be energy dependent. and the energy dependence was observed. The quenching factor we measured here is a little larger than that of previous studies, which may depend on the Eu doping concentration of the crystal. %increase with decreasing recoil %energy, which is an advantage as WIMPs detector. %, which is firstly suggested by the measurement of Davies {et %al.}. %and suggests the interest of CaF$_{2}$(Eu) detectors for dark matter %searches. %It is also suggested that the Eu doping appears to affect the %efficiency for %nuclear recoils. %We compared it to the other studies in terms of the Eu doping concentration.
f-nim-xxx.tex
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%       1st version at September 4, 2000
%       2nd revised October 11, 2000
%       revised after coming back to Seattle, Jan 19, 2001
%	  
%       revised after comments from referee(June 13), June 21, 2001
%	  revised after Kathryn K. Schaffer's reading
%
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\begin{document}
%\runauthor{Cicero, Caesar and Vergil}
%\begin{frontmatter}
%\title{Scintillation efficiency of nuclear recoils in a CaF$_{2}$(Eu) crystal 
%for dark matter search}

\begin{center}

\Large{\bf Scintillation efficiency of nuclear recoils in a CaF$_{2}$(Eu) crystal
for dark matter search \\}

\large
%\thanks[cenpa]{
%%Corresponding author. Fax: +1-206-685-4634. \\
%Present address: Center for Experimental Nuclear Physics and 
%Astrophysics, and Department of Physics, University of Washington, Seattle, WA 98195, %USA. 
%{\em E-mail:} ryutah@u.washington.edu }
%in 5 acts\thanksref{X}}
%%\author[osaka]{R. Hazama\thanksref{jsps}}
%%\author[osaka]{S. Ajimura}
%%\author[osaka]{H. Hayakawa}
%%\author[osaka]{T. Kishimoto}
%%\author[osaka]{K. Matsuoka}
%%\author[osaka]{H. Miyawaki}
%%\author[osaka]{K. Morikubo}
%%\author[osaka]{N. Suzuki}
%\author{R. Hazama\thanksref{cenpa}}, 
%\author{S. Ajimura}, 
%\author{H. Hayakawa}, 
%\author{K. Matsuoka}, 
%\author{H. Miyawaki}, 
%\author{K. Morikubo}, 
%\author{N. Suzuki}, 
%\author{T. Kishimoto}
R. Hazama\footnote{
Present address: Center for Experimental Nuclear Physics and 
Astrophysics, and Department of Physics, University of Washington, Seattle, WA 98195, USA. {\em E-mail:} ryutah@u.washington.edu}, 
S. Ajimura, 
H. Hayakawa, 
K. Matsuoka, 
H. Miyawaki, \\ 
K. Morikubo, 
N. Suzuki, 
T. Kishimoto \\

\normalsize
%\thanks[X]{This is the history of the paper, etc etc}

%\address[osaka]{Department of Physics, Osaka University, Toyonaka, Osaka 560, Japan}
%\address{Department of Physics and Laboratory of Nuclear Studies, \\
%Osaka University, Toyonaka, Osaka 560, Japan}
{\it Department of Physics and Laboratory of Nuclear Studies, \\
Osaka University, Toyonaka, Osaka 560, Japan \\}

\end{center}

%\address[Baiae]{The White House, Baiae}
%\address[Rome]{Senate House, Rome}
%\thanks[jsps]{supported by JSPS Fellowship}
\begin{abstract}
%The energy dependence of t
The scintillation efficiency (quenching factor) of 
fluorine and calcium nuclei recoiling in a CaF$_{2}$(Eu) crystal 
%CaF$_{2}$(Eu) crystal for calcium and fluorine nuclei 
was measured in an energy region relevant to dark matter searches. 
%in 25 $\sim$ 91 keV and 53 $\sim$ 192 keV recoil energy, respectively. 
The recoiling nuclei were obtained via the $^{19}$F(n,n') and 
$^{40}$Ca(n,n') reactions, where the neutron beam was provided 
via the d(d,n)$^3$He reaction. 
%, which gives appropriate low energy recoils. 
%the $^{19}$F(n,n') and $^{40}$Ca(n,n') reaction. 
The quenching factor of F and Ca nuclei in 
the CaF$_2$(Eu) crystal was found to be 
11 $\sim 20 \%$, and 9 $\sim 23 \%$ relative to the electron equivalent 
energy for 53 $\sim$ 192 keV and 25 $\sim$ 91 keV recoil energies, respectively, 
%for the F and Ca nuclei, respectively
%and found to be energy dependent. 
and the energy dependence was observed. 
The quenching factor we measured here is a little larger than 
that of previous studies, which may depend on the Eu doping concentration 
of the crystal. 
%increase with decreasing recoil 
%energy, which is an advantage as WIMPs detector. 
%, which is firstly suggested by the measurement of Davies {\it et %al.}.
%and suggests the interest of CaF$_{2}$(Eu) detectors for dark matter %searches.
%It is also suggested that the Eu doping appears to affect the %efficiency for 
%nuclear recoils. 
%We compared it to the other studies in terms of the Eu doping concentration. 
\end{abstract}

\vspace{1cm}
%\begin{PACS}
%\PACS{PACS number(s): 
\hspace{-0.5cm}{\it PACS:} 29.40.Mc, 95.35.+d, 14.80.Ly. \\
%\end{PACS}
%\begin{keyword}
{\it Keywords:} Scintillation efficiency; CaF$_2$; Dark Matter search; Nuclear recoil \\
%\end{keyword}
%\end{frontmatter}

\section{Introduction}
%hello\footnote{A footnote}
%\subsection{Introduction}
%\subsubsection{Introduction}
%\paragraph{Introduction}
\typeout{SET RUN AUTHOR to \@runauthor}
%\begin{equation}\label{fo:theta}
%  \theta = (a_{1},a_{2},\ldots)^{T}
%\end{equation}

%As first suggested by Goodman and Witten \cite{witten}, searches for %WIMPs 
%have attempted to detect their scattering elastically off nuclei. 
Weakly Interacting Massive Particles (WIMPs), which are major 
candidates for cold dark matter in the universe (especially in conjuction with proposed 
supersymmetric particles \cite{griest}), 
interact rarely with a nucleus. 
When they do, the nucleus receives a recoil energy 
in the 10 $\sim$ 100 keV range by elastic scattering 
via either the spin-independent or the spin-dependent interaction \cite{witten}. 
%the energy is deposited in a crystal by a recoiling nucleus \cite{witten}. 
%The recoil energy by elastic scattering via spin-independent or spin-dependent interaction is in the 10 $\sim$ 100 keV range. 
%This elastic scattering event deposits several keV of kinetic energy %in the nucleus. 
%Since nuclear recoil from such collisions with energies in the keV %range, germanium and silicon semiconductor detectors are used at the %earliest stage. 
%And ultrapure germanium detectors have already ruled out Dirac 
%neutrinos \cite{klap}. 
Compared to coherent (spin-independent) scattering,  
experiments so far have far below the necessary sensitivity to 
exclude the dark matter candidates that have axial vector coupling with matter 
(spin coupled dark matter) \cite{review}.  
Recently several notable advantages of the spin-dependent interaction channel 
have been noticed \cite{collar}\cite{falk}\cite{klap}. 
%such as Majorana neutrinos and neutralinos. 
Calculations by Ellis et al. \cite{ellis} suggest that fluorine 
provides a particularly high elastic scattering rate for WIMPs that 
have axial coupling. 
The rate estimation for $^{19}$F is fairly stable against the 
calculations of nuclear matrix elements, which in some cases vary 
more than an order of magnitude for other nuclei \cite{jungman}. 
%Even though there is a significant spin-independent coupling of neutralinos 
%to nuclei even if the neutralino is a pure state, spin-dependent interaction 
%dominates for nuclei with A $<$ 30 \cite{jungman}. 
We used fluorine-containing CaF$_2$(Eu) scintillator. 
Use of scintillators allows fabrication 
of multi-kg or heavier detectors. 
Experiments to search for rare events require a 
large quantity of material in a low background environment. 
High statistics is especially 
vital for studying the annual modulation, which is 
a unique signature of halo dark matter in our galaxy. 
We developed a new detector system, ELEGANT VI, which consists 
of 25 CaF$_{2}$(Eu) crystals totaling 7.2 kg. 
Details of ELEGANT VI are given elsewhere \cite{km}\cite{wepa}. 

%In a scintillation detector, the nuclear recoil energy is converted into 
%scintillation photons. 
The light output of scintillators for nuclear recoil is 
known to be lower than that for electrons, 
particularly in the low energy region. 
%because only a fraction of the nuclear recoil energy is transferred 
%to electron excitation. 
%%Since slow recoiling nuclei are expected to produce less ionization 
%(electron excitation) than a Compton or photoelectron of the same 
%energy \cite{lind}, these detectors must be calibrated.
%The velocity of the recoil nucleus scattered off by dark mattter is 
%as slow as $v_{R}\sim 10^{-3}c$.
%Thus the ionization (pluse height) of the recoil nucleus is much 
%reduced in comparison with that of the electron with $v_{e}\simeq %c$.
The observed electron equivalent energy (pulse height) $E_{0}$ is given as
\begin{equation}
E^i_0 = f^i E^i_R,
\end{equation}
where $E_{R}$ is the recoil energy and $f$ is the quenching (conversion) factor of the pulse height for a recoiling nucleus ($i$) with 
respect to the electron. 
It is known that the f-value depends largely on scintillators 
and recoiling nuclei. 
%We need to know this $f$-value of our scintillation crystal. 
Although the measurements have been carried out by the 
UK(United Kingdom) Collaboration \cite{UK}\cite{UK98} and the BPRS(Beijing/Paris/Rome/Saclay) 
Collaboration \cite{BPRS} for calcium and fluorine,  
%through neutron-nucleus scattering, 
these measurements are not consistent with each other. 
The difference may be attributed to the characteristics of each 
scintillator, such as 
%%%%%%%%%%%%%% comments from referee %%%%%%%%%%%%%%%%%
%the recoil energy, 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
the activator concentration, 
the configuration, 
and the PMT photocathode sensitivity. 
%efficiency ($f$-value) may depends on the scintillator,  
Therefore we studied one of the 25 crystals that are used for the dark matter 
experiment. 
%; the same mass (dimension), configuration (setup) and Eu doping. 
%Consequently the expected nuclear recoil spectrum is obtained precisely.
%%%%%%%%%%%%%%%%%%%%%
%Such an experiment was done by UK Collaboration \cite{UK}\cite{UK98} and the Beijing/Paris/Rome/Saclay Collaboration \cite{BPRS} for calcium and fluorine through neutron-nucleus scattering. 
%Between these measurements there is a contradiction in the energy 
%dependence of the scintillation efficiency.
%To settle this problem 
%However, these measurements don't come to one conclusion. 
%don't settle down to one conclusion. 
%We performed an experiment which extend 
%the previous results \cite{UK,BPRS} down to recoil energies of 25 
%keV and 53 keV for calcium and fluorine respectively 
%and compared the previous results with respect to the Eu doing. 
%This calibration is vital to evaluate the proper sensitivity in low 
%energy recoil regions.  
%%%%%%%%%%%%%%%%%%%%%
%It is noted here that for this calibration experiment, we used one of the crystal, which is used for the real experiment for dark matter search; the same mass (dimension), configuration (setup) and Eu doping. 
This is essential to evaluate our detector's sensitivity to WIMPs. 
%, if the doping content affect the scintillation efficiency. 
%the meaningful region for dark matter search.  

\section{Experimental setup}

\subsection{Pulsed-monoenergetic neutrons}

The response of the detector to the nuclear recoil of $^{19}$F and $^{40}$Ca 
was measured, where recoil nuclei were produced 
by the $^{19}$F(n,n') and the $^{40}$Ca(n,n') reactions, respectively. 
The experimental setup is schematically shown in Fig.~\ref{setup}. 
A pulsed 0.75 MeV deuteron beam was provided by the 3.2 MV 
Pelletron Accelerator at the Tokyo Institute of Technology. 
The average beam current was 2 $\mu$A at a repetition rate of 2 MHz.
The $\sim$ 3.7 MeV pulsed neutron beam 
%which gives similar energy to a $^{19}$F nucleus as 
%the scattering by the dark matters, 
was obtained by the d(d,n)$^3$He reaction. 
%The $\sim$ 3.7 MeV neutron gives similar energy to a $^{19}$F nucleus as the scattering by the dark matters. 
The neutron flux was about 2$\times$10$^{7}$ /s/sr. 
The energy spectrum of the neutron beam was measured by a time-of-flight (TOF) 
method between the RF signal (the primary target) and a liquid scintillator 
placed in the forward direction. 
This indicated a spread in neutron energy of around 
0.26 MeV (1 $\sigma$). 
%which was effective to discriminate events from those due to thermal 
%neutrons.
%TOF measurements made with a liquid scintillator 
%placed in the forward direction indicated a spread in neutron 
%energy of around 0.26 MeV (1 $\sigma$).
The main contribution to this spread comes from the deuteron energy 
loss in the primary target of Tid$_{2}$. 
The Tid$_{2}$ had a thickness of 0.5 mg/cm$^{2}$ and was made by d$_{2}$ 
gas electrolyzed on a titanium support (which also served as a deuteron beam stop). 
The neutron pulse height response of the BC501A was found to be in agreement 
with the calculation of V. V. Verbinski et al. \cite{verbin}. 
Details of the pulsed neutron beam are described in Ref.~\cite{nagai}. 
%A neutron beam was provided by bombarding a deuterated 
%titanium target with a pulsed 0.75 MeV deuteron beam from a 3.2 MV Pelletron Accelerator at Tokyo Institute of Technology by the d(d,n)$^3$He reaction. 
Neutrons emitted at 0$^{\circ}$ were collimated using a cylindrical paraffin 
collimator, 60 cm in length with a tapered hole along the beam axis. 
The hole was 1 cm in diameter near the primary target and 3 cm in diameter near 
the secondary target, the CaF$_{2}$(Eu) crystal. 

\subsection{\bf The CaF$_2$ detector}

The CaF$_{2}$(Eu) crystal, a 45 mm cube with a europium 
doping content of 0.17 $\%$ by mass 
was placed at the secondary target point. 
%which is one of the 25 crystals used in ELEGANT VI, 
%was used as the target. 
It has a light guide of CaF$_{2}$(pure) crystals coupled to 
photomultipliers \cite{km}\cite{wepa}. 
The CaF$_{2}$(Eu) and CaF$_2$(pure) crystals were made by BICRON. 
The tube is 1.5 inch Hamamatsu H3178 (quartz window).
%although diameter of the socket assembly is reduced to 44.5 mm from %47 mm to install 25 CaF$_{2}$ without any space in between since air %in the space may contain backgrounds for the dark matter search.
%In order to compensate the low light output of CaF$_2$ ($\sim$ 20 photons/keV), the largest PMTs to fit the size of the crytal were utilized to obtain enough light collection efficiency of 60 $\%$ (0.35 keV/photoelectron). 
With this design the CaF$_2$ detector is capable of detecting a low 
energy signal (0.35 keV/photoelectron). 
The whole crystal is covered with four teflon sheets (BICRON BC-642 PTFE 
Reflector tape) and an aluminized mylar 
(Tokyo-metalizing 90313) to maximize the light collection. 
%The light output of CaF$_{2}$ is small ($\sim$20 photons/keV) so we %want to cover maximum area to obtain maximum light collection %efficiency.
%Then we obtained the light collection efficiency of 60 $\%$ (0.35 %keV/photon). 
%One of the crystal in the ELEGANT VI was used for this experiment. 
%The CaF$_{2}$(Eu) detector used here is the same doping as used in %the dark matter detector itself, 
%The CaF$_{2}$(Eu) and pure crystals were made by BICRON. 
%and has been purified to the level about 0.05 ppb for U and 0.24 ppb %for Th.
The linearity of the energy calibration for the CaF$_{2}$(Eu) was 
checked using the $^{241}$Am source and light emitting diode(LED). 
Typical energy resolution as measured with the $^{241}$Am 59.5 keV $\gamma$ ray is about $\bigtriangleup E/E$ = 25 $\%$(FWHM). 
The LED (NLPB500 Nichia Chemical) is a blue LED 
and its maximum fluorescence emission-wavelength is close to 
that of the CaF$_2$(Eu). 
This light is sent to both sides of the crystals through optical fibers (BCF98 BICRON). 
The energy calibration and efficiency of signals below 5 keV are difficult to test 
using a radioactive source, so we generated the pulse shape of the low energy signal 
with the LED connected to a shaper amplifier. 
It is verified that the signal has a similar shape to the real gamma ray signal by using a Flash ADC. 
%We can calibrate the energy scale easily and the stability of energy %during running time. 
The stability of the system was also monitored periodically by 
lighting the LED with a rate of 10 Hz during the experiment. 
%By using the LED signal we observed the pedestal energy shift of 1.6 %keV between beam-on time and beam-off time.
%The signals from the two photomultipliers were separately %discriminated at the level of one photoelectrons, which energy %corresponds to 0.35 keV 
%and circuit noise can be kept below 0.30 keV and then put in %coincidence for triggering purposes.
%We developed low noise circuit and system, which allowed us to %extend our measurements down to the recoil energy of 25 and 53 keV %for Ca and F, respectively. 

\subsection{\bf The neutron detector and NaI detector}

The signal from the CaF$_2$ detector was measured in coincidence with 
scattered neutrons detected by a BICRON BC501A liquid scintillator, 
20 cm $\phi \times$ 5 cm, viewed by a Hamamatsu R4144 photomultiplier. 
This was placed 2 m from the CaF$_{2}$(Eu) crystal with scattering angles 
of 30$^{\circ}$, 40$^{\circ}$, 50$^{\circ}$, and 60$^{\circ}$. 
Kinematics then defines the recoil energy in the target. 
Corresponding recoil energies are 53 $\sim$ 192 keV and 25 $\sim$ 91 keV for F and Ca, respectively. 
The liquid scintillator is sensitive to both $\gamma$-rays and neutrons. 
They were separated by using a standard pulse shape discrimination 
(PSD) method. 
The typical distribution of the total integrated charge over 100 ns (which depends on 
scintillation decay time) versus the initial integrated charge over 26 ns 
(which is proportional to the rise time) recorded by the ADC is shown in Fig.~\ref{spect}(A). 
The two loci correspond to the $\gamma$ and $n$ events. 
The leakage of the $\gamma$ events to $n$ events in the PSD 
selection is less than 1 $\%$ with a $E_{\gamma}$ ($\gamma$ energy) $>$ 0.7 MeV. 
A heavy metal with a thickness of 10 cm was placed between 
the primary Tid$_{2}$ target and the neutron detector to shield 
neutrons and $\gamma$ rays coming directly from the 
Tid$_{2}$ target. 
Accidental coincidence due to the counting rate of the neutron detector became 
tolerable using this shielding. 
Accidental coincidence background was further reduced by the 
pulse height cut of $\gamma$-rays and neutrons in the neutron detector. 
Only events which produced a recoil proton pulse height in the neutron detector with 
a $E_n$ (neutron energy) $>$ 1.2 MeV were included so as to reduce background coincidences, 
which come from detection of scattered neutrons. 
The neutron detector was calibrated with the $^{88}$Y 898 and 1836 keV 
$\gamma$-ray. 
%{\it The neutron pulse height response of the BC501A was found to be in agreement 
%with the calculation of V. V. Verbinski et al. \cite{verbin}.} 
%Then this shielding reduce the counting rate of the 
%neutron detector to limit random coincidences. 

Two NaI(Tl) scintillators, each with the dimensions of 
$85\times95\times95$ mm$^3$ were placed above and below the 
CaF$_{2}$ scintillator to detect escaped $\gamma$-ray events. 
NaI(Tl) detector was calibrated with $^{22}$Na 511 and 
1275 keV $\gamma$-ray. 
The paraffin collimator prevented the NaI(Tl) detector from 
seeing the primary target position directly. 

\subsection{\bf Electronics}

The decay time of the CaF$_2$ detector is quite long ($\sim3 \mu$sec). 
%and the transit time of a pulse through the PMT was about 40 nsec. 
The output pulses from the PMTs were fed into the high-gain 
Timing Filter Amplifier (TFA), which we developed with an RC circuit 
of integration time-constant 200 ns. 
The signals from the two photomultipliers were separately discriminated 
at the level well below one photoelectron. 
%, whose energy corresponds to 0.35 keV and circuit noise can be kept below 0.30 keV. 
Coincidence of two signals from the left and right PMTs with a 
resolving time of 200 ns made a trigger signal. 
%In a few photoelectron level the time jitter of ensemble of one photoelectron were large, then summed pulses were fed again into the TFA 
Pulses from two PMTs were summed and fed again into the TFA 
with an integration time-constant 1 $\mu$s. 
This procedure is particularly useful for a low energy signals 
with a few photons scattered during the long decay time. 
%where the photons are distributed {\bf scatterly} in time. 
%scatter largely in time. 
Then pulses above a threshold of 3 photoelectrons occuring in 
coincidence with neutron events in the neutron counter were accepted. 
We developed a low noise circuit and system and were able to detect signals 
down to 3 photons under a vast amount of noise from the accelerator. 

%, which allowed us to extend 
%our measurements down to the recoil energy of 25 and 53 keV for Ca 
%and F, respectively. 

The pulse heights from the CaF$_{2}$, BC501A, and NaI(Tl) detectors 
and the timing information of the BC501A, CaF$_{2}$, and the deuteron 
beam pulses as determined by an inductive pickup coil were 
recorded event-by-event through CAMAC-SFVME-SUN(Spark Station) on 
a disc. 
%, which was used as the main data acquisition system. 
%A schematic of the electronics is shown in Fig.~\ref{kairo}. 
%%%%%%%%%%%%% cut bb %%%%%%%%%%%%%%%%%%%%%%%%%%
%In this measurements there are three trigger mode such as dark 
%matter mode (DM), double beta mode ($\beta\beta$) and LED mode (LED). 
%The detector is designed to be used also for the search of the
%$\beta\beta$ decay of $^{48}$Ca.  
%The $^{40}$Ca(n,n') reaction was used to excite the first 0$^{+}$
%state (3.35 MeV) in $^{40}$Ca. The 0$^+$ state decays exclusively by
%the emission of e$^+$e$^-$ pair thus energy calibration was carried
%out up to 3.3 MeV \cite{miya}. 
%This reaction was recorded simultaneously in separate 
%analog-to-digital converters (ADC's) and the above three trigger modes 
%was distinguished by using the different logic size of the trigger.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%The required time of measurements depends on the cross section of each 
%scattering angle, but the average time is about 3 hours. 
The trigger rate was typically 
%at the scattering angle of 60$^{\circ}$ degree is 
about 100 cps, which contained a few cps of true signals. 

\section{\bf Results}

%We have applied the following criteria in the off-line 
%analysis to select the true Ca/F recoil events. 
%\begin{itemize}
%\begin{enumerate}
%\item[i)]
%The dark matter (DM) trigger including the rejection of the overflow 
%in the left and right ADC. (The full range of the DM-ADC is about 150 keV.)
%\item[i)]
%\item
Energy spectra for the CaF$_{2}$ detector were obtained in coincidence 
with the neutrons selected by the PSD method. 
%requiring a PSD signature. 
%The high energy $\gamma$ and low energy 
%n cuts in the neutron detector which suppresses the $\gamma\gamma$ 
%and $nn$ random events backgrounds. 
In the WIMPs search, events from the central CaF$_2$(Eu) crystal are selected 
by setting a proper window on the roll-off ratio spectrum \cite{wepa}, 
which gives substantial reduction of backgrounds. 
Roll-off ratio is defined by the relative pulse height difference 
between the left and right PMT to identify the firing position in the 
CaF$_2$ detector. 
The light guide of CaF$_2$(pure) crystals acts 
not only as a light guide but also as an active veto counter for the 
central Eu doped one. 
%In order to obtain the efficiency for the WIMP search, 
We applied the same cut used for the roll-off ratio. 
%A two dimensional plot of both light outputs from the both sides of PMT 
%shows clearly three loci due to three crystals inside the CaF$_2$ detector.
%\item[ii)]
%\item
%CaF$_{2}$(Eu) spectra were obtained by single events requiring only 
%a neutron time-of-flight signature and a pulse shape discrimination (PSD) signature. 
%Accidental coincidence background was substantially reduced by the 
%pulse height cut of $\gamma$-rays and neutrons in the neutron detector. 
%The $\gamma$-rays and low energy n cuts in the neutron detector 
%which suppresses the $\gamma\gamma$ and $nn$ peak and reduces the 
%random events background. 
%The liquid scintillator is sensitive to both $\gamma$-rays and neutrons.%They were separated by using a standard PSD method. 
%The typical distribution of the pulse height which depends on scintillation 
%decay time, versus the pulse height proportional to the rise time, recorded 
%by ADC is shown in Fig.~\ref{spect}. 
%Only events which produced a recoil proton pulse height in the neutron detector with 
%a $E_n$ $>$ 1.2 MeV were included so as to reduce background coincidences, 
%which comes from detection of wall scattered neutrons.
%The neutron detector was calibrated with the $^{88}$Y 898 and 1836 keV 
%$\gamma$-ray. 
%The neutron pulse height response of the BC501A was found to be in agreement 
%with the calculation of V. V. Verbinski et al. \cite{verbin}. 
%\end{itemize}
%\end{enumerate}

An energy spectrum for CaF$_{2}$(Eu) 
%in coincidence with neutron detector at 60$^{\circ}$ is shown in Fig.~\ref{spect}. 
at a neutron scattering angle of 60$^{\circ}$ is shown in Fig.~\ref{spect}(B). 
The two peaks correspond to the Ca and F recoils with energy of 91 keV and of 192 keV, respectively. 
%{\it Similar} energy spectra were observed at each angle. 
The energy spectra were also obtained at 30$^{\circ}$, 40$^{\circ}$, and 50$^{\circ}$. 
The peaks were fitted with a Gaussian 
to determine the mean and the width of the observed energy distribution.
The widths of the Gaussians used in the fit include 
the measured energy resolutions of the detector, 
the uncertainty in the recoil energy which is caused by the 
spread in neutron beam energy, 
and the spread in the scattering angles. 
%In the fitting the measured energy resolutions of the detector, 
%the uncertainty in the recoil energy which is caused by 
%spread in neutron beam energy, 
%and spread in the scattering angles are included for 
%the widths of the Gaussian peaks. 
The relative intensities of the two peaks versus scattering angles 
of neutron are consistent with calculations based on the known elastic 
scattering cross sections. 
It is noted here that the energy threshold of CaF$_2$ detector is only 
a few keV, which is low enough to observe the low recoil energy. 

The fluorine peak has a tail on the high energy side 
in Fig.~\ref{spect}(B). 
The tail may be related to $\gamma$-rays from the 
recoiling nucleus that Compton scatter in 
the target scintillator depositing energy in addition to the recoil energy. 
%$^{19}$F has two low-lying excited levels at 109.9 and 197.2 keV. 
Since the CaF$_2$ is a low $Z$ (atomic number) material, the photoelectric 
effect is of minor importance. 
$^{19}$F has two low-lying excited levels at 109.9 and 197.2 keV. 
%In case of inelastic scattering on $^{19}$F two excited levels are 
%located at low energy (109.9 and 197.2 keV). 
%This tail was slightly reduced by requiring the anticoincidence with 
%the upper and lower NaI(Tl) counters. 
%This was confirmed by using the upper and lower NaI(Tl) counters. 
%Only the events of F recoils are selected by gating the energy region 
%of upper and lower NaI corresponding to the Compton scattering events. 
The differential cross sections for inelastic scattering for these excited 
states are substantial according to the NNDC database \cite{nndc}. 
A simulation with GEANT shows that 21 $\%$ of $\gamma$-rays 
originating from these excited states interact within the crystal 
through Compton scattering. 
%Among these events we can reject the events that escape $\gamma$-ray 
%interact the NaI(Tl) veto counter and the both side 
%pure-CaF$_{2}$ light guide. 
We observed that this tail was reduced by several $\%$ by requiring 
anticoincidence between the upper and lower NaI(Tl) counters. 
This is consistent with an estimate, which includes NaI's acceptance and cross 
sections at each angle. 
%by the cross sections, the simulation, and NaI's acceptance at each angle. 
%This confirmed that the assumption of inelastic events and 
%estimating the peak positions at each angle was not affected much. 
%this effect was allowed for estimating the peak positions at each angle.
%As a result the effect of the inelastic events on the scatter peaks were 
%calculated to be at a level of $\sim 6 \%$  and this was allowed for 
%estimating the peak positions at each angle.

Fig.~\ref{f}(A) shows the measured energy in keV for Ca and F recoils.  
%in CaF$_{2}$(0.17 $\%$ Eu) calibrated relative to electons ($\gamma$-rays) at each energy, versus the corresponding calculated recoil energy. 
The obtained f-values are also shown in Fig.~\ref{f}(B) and (C) together with the previous results of the UK Collaboration \cite{UK98} and the BPRS Collaboration \cite{BPRS}. 
%for an actual purposes of calibrating dark matter detectors. 
%For the purposes of calibrating dark matter detectors a plot of ratio of 
%recoil efficiency to photon efficiency, versus recoil energy, is more 
%useful and this is also given in Fig.~\ref{f} for Ca and F respectively. 
The dominant contributions to the vertical error bars of the present result, 
Fig.~\ref{f}(B) and (C), are 
%statistical and arise from peak finding with poor statistics and from the 
%CaF$_{2}$(Eu) detector resolution.  
due to the CaF$_2$(Eu) detector resolution and the uncertainty in the 
calculated recoil energy, which originates from the beam energy spread 
due to energy loss at the primary target. 
%is due to the experimental geometry and beam energy.
%All errors are included in the statistical error. 

\section{\bf Discussion}

Our result indicates that the f-value of CaF$_2$(Eu) crystal 
%depends on the energy and 
increases with decreasing recoil energy, 
which is consistent with the result of the UK Collaboration \cite{UK}\cite{UK98}. 
On the other hand, the BPRS Collaboration 
\cite{BPRS} suggests that the f-value does not depend on the energy. 
The large f-value at low energy 
%The low energy enhancement 
gives us a good sensitivity as a dark matter experiment.  
A low recoil energy is beneficial for a dark matter search. 
%due to lower energy threshold. 
%So far the scintillation response of CaF$_{2}$(Eu) was measured by the 
%BPRS(Beijing-Paris-Rome-Saclay) Collaboration \cite{BPRS} and 
%the UK(United 
%Kingdom) Collaboration \cite{UK}\cite{UK1}. 
%The former group suggests the f-value does not depend on the energy.
%In contrast to the result of the BPRS group there is evidence 
%from the data 
%of the UK group that the f-value increase with decreasing recoil energy. 
%%To remedy this situation 
%We obtained the data which extend the UK's result down to recoil energies 
%of 25 keV and 53 keV for Ca and F respectively and which are 
%consistent with the UK's result.
%Taking the significant energy dependence of f-values for ionization 
%detectors (Ge and Si) \cite{ge-si} into account, f-values for 
%scintillation detectors may be expected to show the energy dependence. 
%However, the beha 
We applied two assumptions for the dependence of the f-value. 
Generally the f-value is large for particles with small $dE/dX$ (electrons) \cite{hion}. 
We then assume the f-value depends inversely on stopping power, which is represented as: 
\begin{equation}
f \propto \frac {1} {dE/dX} 
\propto \frac{1} {\frac{(dE/dX)_{F(Ca)}}{(dE/dX)_{\beta(\gamma)}}} 
\propto \frac{1}{\frac{(dE/dX)_{F(Ca)}}{\rm minimum-ionization}}. 
\end{equation}
Here $(dE/dX)_{F(Ca)}$ represents the stopping power of $^{19}$F ($^{40}$Ca), 
for which numerical values are tabulated by L. C. Northcliffe 
and R. F. Schilling \cite{ns}. 
%To calculate the f-value for CaF$_2$ detector, total stopping power =
%$S_{tot}= S_n+S_e$ has been evaluated by using current electronic; $S_e$ 
%and nuclear; $S_n$ stopping cross sections \cite{wilson}. 
%In their notation, the reduced nuclear stopping power, 
%$S_n(\epsilon)$, are presented in the form of analytic expressions 
%and $\epsilon$ is a dimensionless measure of the recoil 
%energy(reduced energy) given by 
%\begin{equation}
%\epsilon = E_R \frac {a M_2}{Z_1 Z_2 e^2 (M_1 + M_2)}
%\end{equation}
%where $a$ is the screeing length; $a=0.8853a_0/
%(Z_1^{2/3}+Z_2^{2/3})^{-1/2} (a_0$ is the Bohr radius), $Z_1e$ and 
%$Z_2 e$ are the nuclear charges of the incident and target atoms, 
%respectively and $M_1$ and $M_2$ are the incident and target atom 
%masses, respectively. 
%They suggested the Lindhard et al. results \cite{lindhard} 
%do not apply at very low energy, but are given for comparison 
%purposes for $\epsilon \ge 0.002$. 
%And they also noted that the inclusion of the electronic 
%term; $S_e$ allows a smooth transition to the region $\epsilon >
%\sim 0.1$, where $S_e$ begins to become important.
In Fig.~3(B) and (C), the f-values calculated by eq. (2) 
are compared with the present experimental data. 
%for F recoils in CaF$_2$ and is extrapolated in the low energy regions. 
%for F recoils in Al. 
%Good agreement between the extrapolation and the data gives us 
%confidence that the theoretical predictions should be valid down 
%to several keV. 
%Good agreement between the extrapolation and the data gives us 
%confidence that the above assumption should be valid down 
%to several keV. 
The above assumption gives good agreement with the observed data. 
For the dark matter search we extrapolated the f-value to the low 
energy region as shown in Fig.~3(B) and (C). 
Typically $dE/dX$ starts decreasing below the Bragg-peak, 
which is the energy region of interest. 
We therefore think the extrapolation to the low energy region is justified. 
%between the extrapolation and the data in the low energy. 
Also shown in Fig.~3(B) and (C), for comparison, is a prediction from the Lindhard 
model \cite{lindhard}. 
It gives a good agreement with ionization detectors such as Ge and Si \cite{ge-si}, 
%This does not allow predictions for recoil in CaF$_2$. 
though this is inconsistent with our results. 

%This was considered in terms of the Eu activator concentration of 
%the CaF$_2$(Eu) crystal. 
%In order to see the dependence of activator concentration, 
Our f-value is a little larger than that of the other measurements. 
This may represent a dependence on the Eu activator concentration of 
the CaF$_2$(Eu) crystal. 
We compared our result with the previous experimental results for CaF$_2$(Eu) whose Eu doping content is known \cite{UK}\cite{UK98}. 
%at the recoil energy of 100 keV. 
%P. D. Johnson and F. E. Williams obtained 
The observed f-values for the fluorine nucleus at a recoil energy of 100 keV 
are shown in Fig.~\ref{act}. 
There is no expression 
for the f-value in terms of the activator concentration, 
though a theoretical expression for the 
relative luminescence efficiency as a function of activator concentration 
is given \cite{jw}\cite{ej}\cite{mm}. 
%which was later confirmed by activated inorganic crystals \cite{mm}. 
If we assume that we can apply it to the f-value also, then we obtain 
the curve shown in Fig.~\ref{act}. 
Here the f-value is represented as, 
%the Johnson-Williams function 
%for the f-value based on the fact that the scintillation efficiency can be described by their model for the 
%excitation by x/$\gamma$-rays, $\beta$-rays, and vacuum ultra 
%violet light and the peak efficiency occurs almost similar range of 
%activator concentrations. 
%a fall of efficiency occurs over the same range of activator concentrations for all 
%types of excitation. 
%assuming the following relationship; 
\begin{equation}
f \propto \frac {c(1-c)^z} {c+\sigma_r(1-c)} 
\end{equation}
where $c$ is the mole fraction of the activator Eu, $\sigma_r$ is the ratio of 
the capture cross section for the exciting energy of luminescent at a lattice 
trapping (non-activator) site to that at an activator site, and $z$ is the number of lattice positions surrounding a given activator.  %such that 
%if any one of these sites is occupied by another activator. 
%concentration quenching will occur if another activator atom is contained within $z$. 
%%%%%%%%%%%%%%%% after referee's comment %%%%%%%%%%%%%%%%%%
The parameters for the curve in Fig.~4 are 150 and $2\times10^{-4}$ 
for $z$ and $\sigma_r$, respectively. 
%%Similar result can be also obtained for calcium nucleus 
%%and at the recoil energy of 200 keV. 
Our values for the parameters turns out to be similar to 
those obtained in relative luminescence efficiency measurement of NaI crystal, which 
are 60 and 1$\times10^{-4}$ for $z$ and $\sigma_r$, respectively \cite{mm}.
However, there are 3 parameters including normalization for 3 data points, 
so more data is necessary. 
%{\bf Since there are 3 parameters including normalization for 3 data points, 
%use of eq. (3) may be justified that the present parameter turns out to be 
%similar to those obtained in relative luminescence efficiency of NaI crystal, which 
%%Then, these values are similar to those obtained in relative luminescence 
%%efficiency of NaI crystal, which 
%%In NaI(Tl) case for an excitaion by $\gamma$-rays of energy 662 keV, these parameters 
%are 60 and 1$\times10^{-4}$ for $z$ and $\sigma_r$, respectively \cite{mm}. }
%%%%%%%%%%%%%%%% after referee's comment %%%%%%%%%%%%%%%%%%
%Because of the limitation of the present available data points, the sensitivity of the 
%curve on the parameters of $z$ and $\sigma_r$ is about a factor $\pm$2 in the 
%recoil energy region from 100 keV to 200 keV. 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
This assumption indicates that the f-value of our crystal would have an activator concentration close to that giving the maximum f-value. 
%It is also suggested that the Eu doping appears to affect the efficiency for 
%nuclear recoils. 
%R. B. Murray and A. Meyer calculated scintillation efficiency for $\gamma$-rays 
%of energy 662 keV as a function of activator concentration for activated inorganic 
%crystals \cite{mm}, based on the previous work of P. D. Johnson and . E. Williams %\cite{jw}. 
%as a function of both $dE/dx$ and activator concentration \cite{mm}. 
%This may indicate that for dark matter purposes we should perform calibration 
%experiments using a scintillator with the same doping as used in the dark matter itself,   
%because the activator concentration of the crystals 
%used in the various experiments is surely not the same.
Since there is no clear foundation for using eq. (3) for the f-value,  
further studies are needed. 

\section{\bf Conclusion}

In summary, we observed recoils of calcium and fluorine nuclei 
in the range 25 $\sim$ 91 keV and 53 $\sim$ 192 keV in a CaF$_{2}$(Eu) 
crystal, respectively.
The scintillation efficiency was found to be 11 $\sim 20 \%$, and 9 $\sim 
23 \%$ for F and Ca, respectively.
%The ratio between the ionization produced by a recoil Ca and F nucleus and an 
%electron ranges from 0.23 at 25 keV up to  0.090 at 91 keV and 0.20 at 53 keV 
%up to 0.11 at 192 keV.
These data serve to calibrate a CaF$_{2}$(Eu) dark matter detector whose 
results are discussed in Ref. \cite{wepa}. 

%It is emphasized here that we should pay attention to use 
%the scintillator, which were used for the real experiment 
%for dark matter search on the measurement of scintillation efficiency. 
%This is essential to evaluate the proper sensitivity 
%for WIMPs for each detector. 
%It is emphasized here that we should pay attention to use a 
%scintillator with the same doping as used in the calibration 
%experiments itself for dark matter searches and take the energy 
%dependence into account for a specific scintillator.

%The obtained recoil response is similar to what has been measured for other 
%detectors \cite{response} although our detector is a little bit better than those. 

\begin{center}
\normalsize{\bf ACKNOWLEDGMENTS}
\end{center}
\vspace{15pt}

The authors thank Prof. Y. Nagai, Prof. M. Igashira, Dr. T. Shima, 
the Pelletron Accelerator group of Tokyo Institute of Technology, and Dr. H. Noumi 
for the help during the run, and the Nagai group for their kind support at TIT. 
The authors also thank Prof. Ejiri and Dr. T. Inomata for valuable discussions 
and K. K. Schaffer for careful readings. 
The present work is supported by the Grant-in-Aid of Scientific Research 
and the Ministry of Education, Science and Culture, Japan. 
The work of R. H. and H. M. was supported by JSPS Research 
Fellowships for Young Scientists. 
R. H. would like to acknowledge the hospitality of Nuclear Physics Laboratory, 
University of Washington. 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%\begin{figure}
%\fontfamily{hlcw}\fontshape{it}\selectfont
%\caption{A Song, by William Shakespeare from his play
%  \emph{Cymbeline}, performed by two characters called
%Guiderius and  Arviragus}
%\label{song}
%\end{figure}
%\begin{table*}
%\caption{Defect parameters for AgCl(111) surfaces$^{\#}$}
%\label{defparagcl} 
%\begin{center}
%\begin{tabular}{l l l l}
%\hline 
%$\Delta_j H_{\mathrm{S}}^\circ$, & \multicolumn{2}{c}{Conductivity}& Profile
%analysis\\  
%\cline{2-4}
%$\Delta_j S_{\mathrm{S}}^\circ$ & \cite{S12}& &
%\cite{F4}$^\ast$\\ 
%\hline 
%$\Delta_{\mathrm{i}} H_{\mathrm{S}}^\circ /\mathrm{meV} $ & 370  & 800 & 780\\  
%$\Delta_{\mathrm{i}} S_{\mathrm{S}}^\circ / k $ &  4.7 & 10 &  9.2\\ 
%$\Delta_{\mathrm{v}} H_{\mathrm{S}}^\circ / \mathrm{meV} $ & 880 & 600 & 690 \\ 
%$\Delta_{\mathrm{v}} S_{\mathrm{S}}^\circ / k$ &  2.0 &  0 & 0.6\\ 
%\hline 
%\end{tabular}
%\end{center}
%\vspace*{.6cm}
%\noindent
%$^{\#}$ The evaluation relies on an Ag$_{\mathrm{i}}^\cdot$
%accumulation which is in agreement with the literature but which is still
%under debate (see text) \cite{phdjam,jmjf}. \\
%$^\ast$ A recent reevaluation resulted particularly in different entropy
%values. \cite{S14}.
%\end{table*}

%\begin{figure*}
%Leave 3cm of space for non-electronic graphic on full page width:
%  \vspace{3cm}
%\caption{A Caption}
%\label{f1}
%\end{figure*}

%tutissimo me muro saeptum esse arbitrabor.\footnote{Cicero wrote very
%  long sentences$\ldots$} Quodsi meam spem vis
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

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\newpage
%\appendix
%\section{APPENDIX}
%\begin{figure}
%\caption{see equation \ref{foo}}
%\end{figure}
%\begin{equation}
%a + b = c
%  \label{foo}
%\end{equation}

%{\bf List of figures} 
%%%%%%%%%%%%%%fig 1%%%%%%%%%%%%%%%%%%%%%%
\begin{figure}[h]
%\newlength{\minitwocolumn}
%\setlength{\minitwocolumn}{0.45\textwidth}
%\noindent
%\hspace{0.5cm}
%\begin{minipage}[t]{\minitwocolumn}
%\vspace*{-3.5cm}
%\special{epsfile=/home/hazama/tex/ds/fig/tit_setup_new.epsf vsize=200}
%\centerline{\psfig{file=FIG/tit_setup_new.epsf,height=8cm,width=7cm}}
%\epsfxsize=15cm \epsfysize=10cm \epsfbox{tit_setup_new.epsf}
%\vspace*{7.5cm}
%\end{minipage}
%\vspace{2cm}
\caption[]
%{\bf Fig. 1}
{Schematic top-view of neutron scattering apparatus. 
%Working conditions correspond to d$_{1}$ = 80 cm, d$_{2}$ = 200 cm, 
%$\theta$ from 30$^{\circ}$ to 60$^{\circ}$.}
The neutron collimator was made from a cylindrical paraffin, 
60 cm in length with a tapered hole along the beam axis. 
The hole was 1 cm in diameter near the primary Tid$_2$ target and 3 cm in diameter 
near the secondary target, the CaF$_{2}$(Eu) crystal.
%a 60 cm long, cylinder with a hole along the beam axis, 
%1 cm in diameter near Tid$_{2}$ target and 3 cm in diameter near CaF$_{2}$(Eu) 
%crystal. 
Each scintillator was viewed by a photomultiplier tube (PMT). }
\label{setup}
\end{figure}
%\vspace{0.5cm}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%fig 2%%%%%%%%%%%%%%%%%%%%%%
%\newlength{\minitwocolumn}
%\setlength{\minitwocolumn}{0.45\textwidth}
%\noindent
%\hspace{0.5cm}
%\begin{minipage}[t]{\minitwocolumn}
%\vspace*{2ex}
%\special{epsfile=/home/hazama/tex/ds/fig/tcircuit1.epsf vsize=350}
%\special{epsfile=tcircuit1.epsf vsize=350}
%\epsfxsize=8cm \epsfysize=6.5cm \epsfbox{tcircuit1.epsf}
%\vspace*{8.5cm}
%\end{minipage}

%{\bf Fig. 2(A)}
%Schematic of the electronics chain at experimental room.

%\begin{figure}[hbt]
%\vspace{-3cm}
%\begin{minipage}[t]{7cm}
%\special{epsfile=psd.eps hsize=220}
%\special{epsfile=/home/hazama/tex/ds/fig/ng.eps hsize=220}
%\special{epsfile=FIG/ng.eps hsize=220}
%%%%%%\epsfxsize=7cm \epsfysize=12cm \epsfbox{tcircuit1.epsf}
%\ \vspace*{5cm}
%\end{minipage}
%\begin{minipage}[t]{7cm}
%\special{epsfile=/home/hazama/tex/ds/fig/fca.eps hsize=220}
%\special{epsfile=FIG/fca.eps hsize=220}
%%%%%%%\epsfxsize=7cm \epsfysize=12cm \epsfbox{tcircuit2.epsf}
%\ \vspace*{5cm}
%\end{minipage}
%\end{figure}

%\vspace{0.5cm}

%\setlength{\minitwocolumn}{0.45\textwidth}
%\noindent
%\hspace{0.5cm}
%\begin{minipage}[h]{\minitwocolumn}
%\vspace*{2ex}
%\special{epsfile=/home/hazama/tex/ds/fig/tcircuit2.epsf vsize=350}
%\special{epsfile=FIG/tcircuit2.epsf vsize=350}
%\epsfxsize=8cm \epsfysize=6.5cm \epsfbox{FIG/tcircuit2.epsf}
%\vspace*{10.5cm}
%\end{minipage}

%\caption[]
%\vspace*{2.5cm}
%{\bf Fig. 2(B)}
%{Schematic of the electronics chain} 
%\label{kairo}
%\vspace{0.5cm}
%\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{figure}[hbt]
%\vspace{-3cm}
\begin{minipage}[t]{7cm}
%\special{epsfile=psd.eps hsize=220}
%\special{epsfile=/home/hazama/tex/ds/fig/ng.eps hsize=220}
%\special{epsfile=FIG/ng.eps hsize=220}
%\epsfxsize=7cm \epsfysize=7cm \epsfbox{ng.eps}
%\ \vspace*{5cm}
\end{minipage}
\begin{minipage}[t]{7cm}
%\special{epsfile=/home/hazama/tex/ds/fig/fca.eps hsize=220}
%\special{epsfile=FIG/fca.eps hsize=220}
%\epsfxsize=7cm \epsfysize=7cm \epsfbox{fca.eps}
%\ \vspace*{5cm}
\end{minipage}

%\end{figure}
%\hspace{1cm}

%\vspace{2cm}
\caption[]
%{\bf Fig. 3} 
{(A): 
The typical distribution of the total integrated charge over 100 ns (which depends on 
scintillation decay time) versus the initial integrated charge over 26 ns 
(which is proportional to the rise time) recorded by the ADC. 
%The typical distribution of the total integrated charge of 100 ns 
%which depends on scintillation decay time, versus the initial 
%integrated charge of 26 ns proportional 
%to the rise time, recorded by ADC. \\
(B): 
An energy spectrum of CaF$_{2}$(Eu) 
%in coincidence with neutron detector at 60$^{\circ}$ is shown in Fig.~\ref{spect}. 
at a neutron scattering angle of 60$^{\circ}$. 
The two peaks correspond to Ca and F recoils 
with energy of 91 keV and 192 keV, respectively.} 
\label{spect}
\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{figure}[hbt]
%\vspace*{-2cm}
\begin{minipage}[t]{7cm}
%\special{epsfile=/home/hazama/tex/effi.eps hsize=220}
%\special{epsfile=effi.eps hsize=220}
%\epsfxsize=7cm \epsfysize=6cm \epsfbox{effi.eps}
%\ \vspace*{5cm}
\end{minipage}
\hspace{1cm}
\begin{minipage}[t]{7cm}
%\special{epsfile=reco.eps hsize=220}
%\epsfxsize=7cm \epsfysize=6cm \epsfbox{reco.eps}
%\special{epsfile=/home/hazama/caf2/dm/topdraw/tit/reco.eps hsize=220}
%\special{epsfile=/home/hazama/tex/ds/fig/reco.eps hsize=220}
%\ \vspace*{5cm}
\end{minipage}
%\end{figure}

%\vspace{1cm}
\caption[]
%{\bf Fig. 4} 
{(A): 
The measured energy in keV for Ca and F recoils in 
CaF$_{2}$(Eu) calibrated relative to electons ($\gamma$-rays) 
at each energy, versus the corresponding calculated recoil energy. 
The errors are statistical only. \\
(B) and (C): 
%The ratio of recoil efficiency to photon efficiency, 
The f-values (quenching factors) 
versus recoil energy in CaF$_{2}$(Eu) for (B) Ca recoils and (C) F recoils. 
The crosses are the data from the present experiment, 
diamonds and stars are data points from the UK Collaboration \cite{UK98} 
and the BPRS Collaboration \cite{BPRS}, respectively.  
The f-values are not plotted by the BPRS Collaboration \cite{BPRS}, so 
we converted by taking only their vertical (statistical) error bar into account. 
The systematic errors are also included for the UK Collaboration and ours. 
%The errors for the BPRS data are statistical only. 
The dashed line is the extrapolation of the f-values calculated by eq.~(2) 
and Ref. \cite{ns} (squares). 
The solid curve is the prediction from the Lindhard model \cite{lindhard}.} 
\label{f}
\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%fig 5%%%%%%%%%%%%%%%%%%%%%%
\begin{figure}[hbt]
%\vspace*{2cm}
%\newlength{\minitwocolumn}
%\setlength{\minitwocolumn}{0.45\textwidth}
%\noindent
\hspace{2cm}
%\begin{minipage}[t]{\minitwocolumn}
%\vspace*{-2.5cm}
%\special{epsfile=/home/hazama/tex/ds/fig/tit_setup_new.epsf vsize=200}
%\centerline{\psfig{file=FIG/tit_setup_new.epsf,height=8cm,width=7cm}}
%\epsfxsize=8cm \epsfysize=8cm \epsfbox{act.eps}
%\vspace*{7.5cm}
%\end{minipage}
%\vspace{2cm}
\caption[]
%{\bf Fig. 1}
{The f-values as a function of Eu concentration (mole fraction) for fluorine nucleus  
in CaF$_2$(Eu). 
Experimental data from Ref. \cite{UK}(open circles) and this work(full 
circle) are shown, and the errors are only statistical. 
The dashed line is obtained from eq.~(3). }
\label{act}
\end{figure}
%\vspace{0.5cm}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newpage
\begin{figure}[hbt]
\vspace{14cm}
\hspace{-1cm}
%\epsfxsize=15cm \epsfysize=10cm \epsfbox{tit_setup_new.epsf}
%\special{epsfile=/Home/pchome/hazama/TEX/DM/TIT/tit_setup_new.epsf %vsize=230}
%\special{eps:tit-setup_new1.eps x=18cm, y=10cm}
\epsfxsize=18cm \epsfysize=10cm \epsfbox{tit-setup_new1.eps}
%\caption[]
%\ \vspace*{2cm}
%{\bf Fig. 1}
\end{figure}
\vspace*{1cm}
{\bf Fig. 1}

\newpage
\begin{figure}[hbt]
\vspace{18cm}
%\epsfxsize=7cm \epsfysize=7cm \epsfbox{ng.eps}
%\special{epsfile=/Home/pchome/hazama/TEX/DM/TIT/ng.eps hsize=500}
%\special{eps:ng.eps x=15cm, y=15cm}
\epsfxsize=15cm \epsfysize=15cm \epsfbox{ng.eps}
%%\caption[]
\vspace*{1cm}
{\bf Fig. 2(A)}
\end{figure}

\newpage
\begin{figure}[hbt]
\vspace{18cm}
%\epsfxsize=7cm \epsfysize=7cm \epsfbox{fca.eps}
%\special{epsfile=/Home/pchome/hazama/TEX/DM/TIT/fca.eps hsize=500}
%\special{eps:fca.eps x=15cm, y=15cm}
\epsfxsize=15cm \epsfysize=15cm \epsfbox{fca.eps}
%%\caption[]
\vspace*{1cm}
{\bf Fig. 2(B)}
\end{figure}

\newpage
\begin{figure}[hbt]
%\hspace*{-2cm}{
\vspace{18cm}
%\epsfxsize=7cm \epsfysize=6cm \epsfbox{effi.eps}}
%\special{epsfile=/Home/pchome/hazama/TEX/DM/TIT/effi.eps hsize=500}}
%\special{eps:effi.eps x=17cm, y=14cm}
\epsfxsize=17cm \epsfysize=14cm \epsfbox{effi.eps}
%%\caption[]
%%\vspace*{28cm}
%%{\bf Fig. 3(A)}
\end{figure}
\vspace*{1cm}
{\bf Fig. 3(A)}

\newpage
\begin{figure}[hbt]
%\hspace*{-2cm}
\vspace{18cm}
%\epsfxsize=7cm \epsfysize=6cm \epsfbox{reco98.eps}
%\special{epsfile=/Home/pchome/hazama/TEX/DM/TIT/reco.eps hsize=500}
%\special{eps:reco98-ca.eps x=17cm, y=14cm}
\epsfxsize=17cm \epsfysize=14cm \epsfbox{reco98-ca.eps}
%%\caption[]
%%\vspace*{18cm}
%%{\bf Fig. 3(Right)}
\end{figure}
\vspace*{1cm}
{\bf Fig. 3(B)}

\newpage
\begin{figure}[hbt]
%\hspace*{-2cm}
\vspace{18cm}
%\epsfxsize=7cm \epsfysize=6cm \epsfbox{reco98-f.eps}
%\special{epsfile=/Home/pchome/hazama/TEX/DM/TIT/reco.eps hsize=500}
%\special{eps:reco98-f.eps x=17cm, y=14cm}
\epsfxsize=17cm \epsfysize=14cm \epsfbox{reco98-f.eps}
%%\caption[]
%%\vspace*{18cm}
%%{\bf Fig. 3(Right)}
\end{figure}
\vspace*{1cm}
{\bf Fig. 3(C)}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%\newpage
%\begin{figure}[hbt]
%%\hspace*{-2cm}
%\vspace{18cm}
%%\epsfxsize=7cm \epsfysize=6cm \epsfbox{reco98.eps}
%%\special{epsfile=/Home/pchome/hazama/TEX/DM/TIT/reco.eps hsize=500}
%\special{eps:reco98.eps x=17cm, y=14cm}
%%\caption[]
%%\vspace*{18cm}
%%{\bf Fig. 3(Right)}
%\end{figure}
%\vspace*{1cm}
%{\bf Fig. 3(D)}

\newpage
\begin{figure}[hbt]
\vspace{18cm}
%\epsfxsize=8cm \epsfysize=8cm \epsfbox{act.eps}
%\special{epsfile=/Home/pchome/hazama/TEX/DM/TIT/act.eps hsize=500}
%\special{eps:act.eps x=15cm, y=15cm}
\epsfxsize=15cm \epsfysize=15cm \epsfbox{act.eps}
%%\caption[]
\vspace*{1cm}
{\bf Fig. 4}
\end{figure}



\end{document}

%\subtitle{Romano dignitas salusque pariatur.
%Ego sum ille consul, patres
%conscripti, cui non forum, iu quo omnis aequitas continetur, non campus
%consularibus auspiciis consecratus, non curia, summum auxilium omnium
%gentium, non domus, commune perfugium, non lectus ad quietem datus, non
%denique haec sedes honoris [sella curulis] umquam vacua mortis periculo
%atque insidiis fuit.  Ego multa tacui, multa pertuli, multa concessi.
%}
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