Introduction

The main purpose of the Barrel Scintillation Detector (BSD) is to increase the acceptance of the Radphi experiment for the slow protons recoiled from $\gamma p$ interactions. In combination with the Barrel Gamma Veto (BGV) detector, which wraps around the BSD, it can be used to increase the acceptance for photons. The BSD detector consists of 48 scintillating paddles, with attached PMTs, grouped in three layers. The inner two layers have 12 paddles each, organized as right-hand (the first) and left-hand spirals (the second layer), while the third set consists of 24 straight paddles. The scintillating paddles are numbered, according to their layer, as BSD channels R0 - R11, L12 - L23 and S24 - S47. Coincidence between overlapping BSD channels forms a pixel. Coordinates of pixels are defined by straight paddle and pixel index. Pixel index is numbered 0 - 7 from upstream to downstream. These two coordinates allow us to form spatial distributions of detected particles.

In this technical note we analyze the analog (adc) signal from the BSD detector. For the given layer, we should not expect that signal above some well defined threshold depends on the counter number (azimuth dependence). The way to ensure this is to match relative gain within layers. In the next section the matching procedure is explained and the gain constants used to match relative gains are given. Analysis of the adc signal within particular BSD counter shows that average gain decreases as pixel index increases (polar angle dependence). One interpretation is that signal is decreasing because of attenuation of light along a BSD counter. In the last section this assumption is justified and the pixel gain distributions, before and after attenuation correction, are presented.