Beam Test by a Real-size Prototype Drift Chamber

A real-size prototype of Belle CDC was constructed and used in a beam test to study the gas-gain saturation effect in $dE/dx$ measurements. The chamber is 235 cm long, and the inner and outer radii are 25 cm to 90 cm, respectively. It has 41 layers of sense wires, 9 layers fewer than the final design [36]. A beam test was carried out at the $\pi$2 beam line. The chamber was placed in a null magnetic field. The drift time was measured by a LeCroy FASTBUS TDC 1876 and the total charge was integrated by a LeCroy FASTBUS 1885F. The measured spatial resolution was 120 $\sim$ 150 $\mu$m depending on the layer and incident angles in the direction perpendicular to the wires.
For the $dE/dx$ measurement we took the truncated mean in order to minimize the contribution of the Landau tail in the $dE/dx$ distribution. In the analysis of this section 80 % truncated means are used to measure $dE/dx$. The $dE/dx$ resolution was obtained to be 5.2 % for 3.5 GeV/c pions at an incident angle of $45^o$. Fig. shows measured $dE/dx$ distributions as a function of $\beta\gamma$. The solid curve is a fit to the data at an angle of $45^o$, based on the most-probable energy loss formula [36]. The density correction term $\delta$ was parameterized to fit the electron data. The dashed curve corresponds to the case of $\delta$ = 0.

Figure: (a) Measured $dE/dx$ vs. $\beta\gamma$ and (b) the same as (a),but normalized with the $dE/dx$ measured by 3.5 GeV/c protons ($\beta\gamma$ = 3.73). The solid curve is a fit to the data at an incident angle of 45$^o$, and the dashed curve is that with $\delta$ = 0.

The incident angle dependence of $dE/dx$ was measured. The normalized $dE/dx$ distributions to $dE/dx$ values calculated from the fit obtained for the incident angle of $45^o$ decrease with increasing incident angles. This indicates a clear space charge effect. The gas-gain saturation effect is mainly determined by the projected ion pair density produced by the incident particles. Fig. shows distributions of $(dE/dx)_{meas.}/(dE/dx)_{expect}$ vs. $(dE/dx)_{meas.}/\cos \theta$ for various beam conditions. The $x$-axis corresponds to the projected ion pair density and the magnitude of the $y$-axis is essentially independent of incident particles.

Figure: (a) $(dE/dx)_{meas}/(dE/dx)_{expect}$ vs. $(dE/dx)_{meas}/cos\theta$ (avalanche density on the sense wire) for 0.8 GeV/c protons, 0.8 GeV/c pions, 0.6 GeV/c electrons and 3.5 GeV/c pions, and (b) $(dE/dx)_{meas}/(dE/dx)_{expect}$ vs. $(dE/dx)_{meas}/cos\theta$ for 0.6, 0.7 and 1.0 GeV/c protons and 2.0 GeV/c electrons. The solid curves are the fit results. The 90$^o$ data are treated separately.

It is important to correct data for the space charge effect to maintain the particle identification performance expected from the obtained resolution and $\beta\gamma$ dependence of $dE/dx$.