Detector Design

The DAPHNE detector [1] already meets most of the requirements for the experimental check of the GDH sum rule at Mainz [2]. However we considered important to extend the DAPHNE acceptance (21 $^\circ \le \vartheta \le$ 159 $^\circ$ ; 0 $^\circ \le \varphi \le$ 360 $^\circ$ ) as close as possible to 4 $\pi$ sr in order to minimize the systematic errors in the cross section extrapolation.

Because of the severe constraints induced by the DAPHNE mechanical frame, two different devices have been realized. The MIDAS set-up [3] covers the intermediate forward region ( $7.5^\circ < \vartheta < 16.5^\circ$ ), while a scintillator-lead device is placed externally to the DAPHNE frame covering the region $\vartheta < 5^\circ$ [4].

Between the two devices a Cerenkov aerogel detector enables electron detection with a threshold of about 10 MeV and an efficiency

99%. Its signal is used as veto to suppress the electromagnetic background on-line.

The volume in front of the target and available for MIDAS is very limited. Therefore silicon detectors were chosen in order to obtain a compact device.

**Figure 1:** Side view of the device
$\begin{figure} \begin{center} \epsfig {file=midas.eps, width=14cm} \end{center} \end{figure}$

MIDAS includes two parts (fig. 1): a tracking section consisting of two double sided silicon detectors (V

and V

) and a double silicon/lead sandwich (Q

, Pb, Q

) for energy measurements.

The 5 detectors have a diameter of about 80 mm and a thickness of 1000 $\mu$ m. A central hole let the primary $\gamma$ beam go through.

The p-side of the tracking detectors is divided into 48 concentric rings, while the n-side is segmented into 16 radial sectors. The signals are carried out by a flexi-rigid cable consisting of 5 overlapping layers screened one to each other by a grounded wire-netting structure. The cable is terminated by 3 50-pins connectors and its measured capacitance is 222 pF/m.

The remaining detectors Q

, Q

and Q

are single sided with the p-side segmented into 4 radial sectors (quadrants).

**Table 1:** Detector characteristics
	Tracking Detectors	Quadrant Detectors
Thickness	1000 $\mu m$	1000 $\mu m$
Geometry	Annular Ring	Annular Ring
inner radius	11 mm	14 mm
outer radius	35 mm	43 mm
Type	Double sided	Single sided
p side	48 rings	4 sectors
n side	16 sectors	-

The silicon detectors were manufactured by Micron Semiconductor Ltd, UK. Their main geometrical parameters are listed in table 1.

**Figure 2:** Layout of the MIDAS set-up
$\begin{figure} \begin{center} \epsfig {file=tube.eps, width=12cm}\end{center}\end{figure}$

The mechanics has been entirely designed and realized by the I.N.F.N. Pool of Pavia (fig. 2). The detectors and the lead absorbers are mounted inside an Aluminum tube fitting into the forward hole of the DAPHNE detector. The detector signal cables are distributed all around the inner surface of the tube. They are guided by a cylindrical basket and they exit from the back opening of the tube. Such a cable system lets the $\vartheta < 5^\circ$ region free for the particle transmission towards the other forward devices.

The tube is closed at both ends by two Aluminum windows, 25 $\mu m$ thick. It is also provided with a gas inlet and a gas outlet to supply an Argon flow of few $\ell$ /hour. In such a way the detectors current is very stable and does not depend on the atmosphere impurities or humidity.