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\title{Cherenkov detector for measuring the $^{90}$Sr activity using silica aerogel}
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%\subtitle{\red{h}}% N.B. can be redefined throughout document%
\author{ {\red Rok Pestotnik},  S.~Korpar, P.~Kri\v zan, A.~Stanovnik}
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\address{  Jo\v zef Stefan Institute, Univ. of Ljubljana, Univ. of Maribor, Slovenia}
%Faculty of Chemistry and Chemical Engineering, University of Maribor, Slovenia 
%Faculty of Mathematics and Physics, University of Ljubljana,  Slovenia 
%Faculty of Electrical Engineering, University of Ljubljana,  Slovenia}
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\date{April 9,  2003}
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\conference{5th symposium of the croatian radiation protection association}
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\newcommand{\sqbox}[2]
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    {\getcolor[slsc]{#1}\color{#1} #2}}
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    {\getcolor[slsc]{#1} \color{#3} #4}}

\newcommand{\ho}[1]{{\color{red} {#1}}}
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\begin{document}
\begin{slide}
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\vfil
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\begin{itemize}
  \item Motivation
  \item Introduction: Cherenkov radiation
  \item Cherenkov radiator
  \item Photon detector
  \item The apparatus
  \item Measurements and Results
  \item Conclusions
\end{itemize}
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\vfil
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\end{slide}



\begin{Slide}{Motivation}




\begin{itemize}
\item $^{90}Sr$ produced as a fission byproduct of uranium and plutonium.
\item It was released in the environment as a consequence of
\begin {itemize}
\item nuclear tests since 1945,
\item nuclear plant accidents (e.g. Chernobyl),
\item nuclear waste discharges.
\end{itemize}
\item   $^{90}Sr$ is a component of contaminated soils at radioactively contaminated sites
\item Environmental concentrations are low, but can become part of the food chain
\includegraphics[width=7cm]{pic/food_chain.eps}

\item It has long physical (28.6 y) and biological (49.3 y) half-life.
\item  $^{90}Sr$ is highly radio-toxic: chemically similar to  $Ca$. Replaces $Ca$ in bones and teeth.
\end{itemize}


{\color{red}\tick  To block the inflow of  $^{90}Sr$ into human a food (meat) quality assessment is needed.}

\end{Slide}



\begin{Slide}{$^{90}$Sr detection}
The $^{90}Sr$ accumulates in bones. Detection in meat indirectly by measuring the activity in bones. 

 {\color{red} Problem}:\ding{54}  $^{90}Sr$  is a pure $\beta$ emitter:

\centerline{\includegraphics*[height=3cm]{pic/razpad}\hskip 2cm \includegraphics*[height=3cm]{pic/srspekter}}

\begin {itemize}
%$^{90}Sr\to ^{90}Y + e^- (E_{max}=0.55~MeV)$
%$$\to ^{90}Zr + e^- (E_{max}=2.27~MeV)$$
\item Standard detection methods using $\gamma$- spectrometry are not possible.
\item Chemical analysis: separation of $Sr$ is time consuming and has  large error
\end{itemize}


{\color{red} Recent analyzes}:
\begin {itemize}
\item use Cherenkov radiation in aqueous solutions
\item  {\color{red} Problem}: Other $\beta$ radioactive components have max. $\beta$ energy around 1.5~MeV and below.
\ding{54} Their Cherenkov photon yield interfere with photons produced by $\beta$ electrons from $^{90}Sr$.

\end{itemize}

 {\color{red} Solution}:
\begin{itemize}

\item {\color{red} Important difference}: The energy of the $\beta$ electrons from $^{90}Sr$  is on average higher than energy of electrons from background elements.
\item {\color{red}Idea}: employ Cherenkov radiation in aerogel and discriminate electrons with different energies.
\end{itemize}



\end{Slide}



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\begin{Slide}{Cherenkov radiation}

Charged particles with a velocity greater than velocity of the light in the medium emit Cherenkov radiation

\doublecolumn{0.50\linewidth}
{
 \mysqbox{red}{red}{white}{Properties:}

\begin{itemize}
\item {Cherenkov  angle:\hskip 0.3cm  \mysqbox{red}{green}{blue}{$\cos\theta_c={ 1 \over \beta n(E)}$}}
\item {Threshold: \hbox{$\cos\theta_C \le 1$ $\Rightarrow$  \mysqbox{red}{green}{blue}{$\beta > \beta_t={ 1 \over n}$}}}

\item Number of emitted photons \\ per unit length and energy: \\
 \mysqbox{red}{green}{blue}{${d^2N \over dl dE}={\alpha\over \hbar c}\sin^2\theta={\alpha\over \hbar c}\biggl( 1-{1\over (n\beta)^2}\biggl)$}
$${\alpha\over \hbar c}=370 (eV\,cm)^{-1}$$
\item  {\color{red} \v Cerenkov detectors } employ the properties of Cherenkov radiation.
\end{itemize}

}
{
\includegraphics[width=6cm]{pic/cerenkovo_sevanje}\\
%\ho{\tick kinemati\v cno obmo\v cje  dolo\v ca sevalec}
%\includegraphics[width=7cm]{aerogel_ndet_vs_p}

}


%Number of detected photons:
%{\color{red} $ N_{det} = L { \alpha \over \hbar c} \int { T(E)  \cdot {\epsilon}_{d}(E)  \cdot \sin^2\theta  \,   dE}$}


\hbox{\sqbox{blue}{Number of detected  photons:}  \mysqbox{red}{green}{blue}{ $ N_{det} = L { \alpha \over \hbar c} \int { T_{r}(E) \cdot {\epsilon}_{d}(E)  \cdot \sin^2\theta_c  \   dE}$ }} 

\vskip 1cm

\hbox{
\hskip 0.15cm
\mysqbox{red}{green}{blue}{\small Transmission} \psline[linewidth=2pt,linecolor=blue]{->}(0,0)(4.5,1.3)
\hskip 0.5cm
\mysqbox{red}{green}{blue}{\small Detection efficiency} \psline[linewidth=2pt,linecolor=blue]{->}(0,0)(2.2,1.3)
\hskip 0.5cm
\mysqbox{red}{green}{blue}{\small $\theta_c$} \psline[linewidth=2pt,linecolor=blue]{->}(0,0)(2.5,1.3)
\hskip 0.5cm
\mysqbox{red}{green}{blue}{\small Energy interval} \psline[linewidth=2pt,linecolor=blue]{->}(0,0)(+0.5,1.3)
}

\end{Slide}



%**************************************************************************
\begin{Slide}{Cherenkov Radiator}

\small

Requirements: \hfil \hbox{\mysqbox{red}{green}{blue}{ $ N_{det} = L { \alpha \over \hbar c} \int { T_{r}(E) \cdot {\epsilon}_{d}(E)  \cdot \sin^2\theta_c  \   dE}$ }}
\begin{itemize}
%\item {$\Delta \theta_c \approx {1 \over n^2 \theta_c}\Delta\beta$}
\item Maximize photon yield $\blacktriangleright$
\begin{itemize}
\item {\red maximize refractive index $n$ } 
\item maximize photon collection area  ${\epsilon}_{geo}(E)$
\item maximize photon detector sensitivity $Q.E.$

\end{itemize}
\item  Minimize photon yield for low energy electrons  $\blacktriangleright$  {\red minimize refractive index $n$ } 
\end{itemize}


\ding{74} With appropriate choice of  refractive index $n$, the background is decreased.


\includegraphics*[height=4cm]{pic/aerogel_thc_e}
\includegraphics*[height=4cm]{pic/srspekter}

\ding{74} \mysqbox{red}{green}{blue}{Silica aerogel radiator meets the requirements.} Fills the gap between solid and gas radiators.
\begin{itemize}
\color{blue}
\item \color{blue} The aerogels with different refractive indexes can be produced
\end{itemize}

\end{Slide}

\begin{Slide}{Aerogel Properties}
\small 
\begin{itemize}

\item Consists of grains of amorphous SiO$_2$ with sizes 1-10~nm linked together in a 3D structure filled by air
\item As a result it is a highly porous material with different pore sizes (mean pore diameter 20~nm) .
\item High surface area material (up to 1000~$m^2/g$) 
\item Produced by the supercritical extraction of the pore fluid
\item variable density: $\rho=0.003- 0.55 g/cm^3$ (controlled by starting silicon compound concentration)
\item  \mysqbox{red}{green}{blue}{variable refractive index} ($n=1+0.21 \rho$): 1.0006 - 1.011
\item \mysqbox{red}{green}{blue}{Rayleigh scattering}: macro-pores act as scattering centers $T=Ae^{Ct\over \lambda^4}$
%\hskip -0.5cm 
\includegraphics[height=4cm]{aeroblock}
\hskip 2cm
\includegraphics[height=4cm]{pic/aerogel_structure}

\ding{74} \mysqbox{red}{green}{blue}{Highly transparent aerogel tiles of thicknesses up to 5.5~cm were produced recently.}
\item Primarily it is hydrophilic.
\item Can be made hydrophobic: replace hydrophilic $-OH$ groups on the internal aerogel surface with hydrophobic ones (lower transparency)
\end{itemize}

\end{Slide}

\begin{Slide}{Photon Detector choice}

Requirements: \hfil \hbox{\mysqbox{red}{green}{blue}{ $ N_{det} = L { \alpha \over \hbar c} \int { T_{r}(E) \cdot {\epsilon}_{d}(E)  \cdot \sin^2\theta_c  \   dE}$ }}

\begin{itemize}
\item high Q.E.
\item position sensitive
\item good single photon resolution
\item stable
\end{itemize}

Based on experiences, we have chosen Hamamatsu R5900-M16 multi-anode PMT.
\begin{itemize}
\item bi-alkali photo-cathode: Q.E. - up to 25 \% at 400~nm
\item segmented anode: active area  18~mm$\times$18~mm with 16 pads 4.5~mm$\times$4.5~mm each   
\item good single photon resolution
\includegraphics[height=4cm]{pic/pulseheight}\includegraphics[height=4cm]{pic/2dscan}

\end{itemize}

\end{Slide}


\begin{Slide}{Measurements}

\small 
\mysqbox{red}{green}{blue}{Detection of $^{90}Sr$ in animal bones}
\vskip 1cm
\doublecolumn{0.30\linewidth}
{
 \mysqbox{red}{red}{white}{The apparatus}
\begin{itemize}
\item light tight box
\item source or sample
\item multiwire proportional chamber
\item Silica aerogel tile (n=1.05)   5.5$\times$5.5$\times$5~cm$^3$
\item reflective entrance and side walls (mylar) to collect photons on the detector
\item Multi-anode PMT
\item electronic readout: VME
\end{itemize}
}
{
\includegraphics[width=8cm]{pic/srsetup}
}

\mysqbox{red}{red}{white}{Data acquisition:}
\begin{itemize}

\item Trigger: Multi wire proportional chamber to lower the background of $\gamma$ emitters
\item Data acquisition: TDC spectra of all electronic channels acquired by CAEN VME V673A TDC 
\end{itemize}

\end{Slide}


\begin{Slide}{Results:}
 
\doublecolumn{0.50\linewidth}
{



\begin{itemize}
\item the background rate less than 100/s for PMT and for MWPC leading to
\item {\color{red} the coincidence background rate $<$ 2/hour}
\item {\color{red} the detection efficiency $1.5\% $ for $^{90}Sr/^{90}Y$}
%\item different radioactive sources were used to determine efficiency

%\item by using larger photon detector higher efficiency is expected
\item as a background to Sr-90,the elements  Cs-137 and P-32 were used
\item $\beta-\gamma$ cascade might trigger a coincidence: the low energy $\beta$ triggers the MWPC, $\gamma$ is converted in the PMT window, the resulting electron emits Cherenkov photons   
\end{itemize}




\vskip 1cm
\begin{itemize}
\item contribution of different sources can be separated by $\to$ 
\item counting the number of photons produced by $\beta$ electron
\end{itemize}



}
{
\includegraphics*[width=7cm]{pic/srefficiency}
\vskip -1.0cm
\includegraphics*[width=7cm]{pic/5gelnhit_e}
}




\end{Slide}


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\begin{Slide}{Conclusions}

\begin{itemize}
\item  $^{90}Sr$ is highly radio-toxic
\item It enters human through the food chain
\item By controlling the concentration of $^{90}Sr$ in animal bones one can block the inflow channel.
\item Standard $\gamma$ spectroscopy impossible since it is  a pure $\beta$ emitter
\item It has relatively high energetic decaying products.
\item Cherenkov radiation in silica aerogel can be used for low activity  measurements 
\item {\color{red}\tick Method was tested and proven that  works}
\end{itemize}

Foreseen activities:
\begin{itemize}
\item test on samples extracted from animal bones
\item geometry optimization 
\item use of the photon detector (flat panel PMT) which covers larger area ( 49$\times$49 mm$^2$) to collect more photons
\includegraphics*[width=4cm]{H8500.eps}
\item aerogel optimization (use more transparent samples)
\end{itemize}

\end{Slide}
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\end{document}