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ReportOnFirstPionTestExperiment

 
The presently available pion ($\pi^{\pm}$) beam at GSI has opened a new possibility to perform $\gamma$ - spectroscopy of hypernuclei with a high resolution germanium array. Till the date very few experiments have been performed with germanium arrays to investigate in detail hypernuclear structure, the spin dependent $\lambda$ - nucleus interaction, etc. This information is needed to constrain the $\lambda$ - $\sigma$ and $\lambda$ - $\omega$ coupling constants and to test the current meson exchange and quark models of hyperon - nucleon interaction.

A test experiment has been performed to investigate the feasibility of such study using the newly developed $\pi^+$ beam facility and germanium crystals at GSI (see proposal S234). The $\pi^+$ beam was produced by impinging a 1.5 GeV/u $^{12}$C beam on a 100 mm long pencil-like Be target. The secondary $\pi^+$ beam was transported to cave C (kaon spectrometer) after passing through different  focusing elements and tracking detectors.

Figure 1 : Time-of-flight spectrum for $\pi^{+}$ and protons. The first peak corresponds to protons, the second one to $\pi^{+}$.

 

A large area 1 cm thick scintillator (HODOSCOPE) detector was placed right after pion production target. A similar detector (but also X-Y position sensitive) will be required in the final experiment for the beam tracking. Another large area scintillator was placed at the reaction target position ($\sim$ 50 m from the production target). Two large area scintillators were placed at a distance $\sim$ 2 m behind the reaction target. A time-of-flight method was used for separation of $\pi^+$ from other charge particles, taking the timing between the large scintillator at the reaction target position and one of the scintillators behind the target. A typical time-of-flight spectrum is shown in fig.1.
 
 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Figure 2 : The $\pi^+$ intensity per spill for different thicknesses and different target materials. The secondary beam momentum was 1 GeV/c

 

 The pion production rate was measured for different production target and different secondary beam momenta. Figure 2 shows the dependence of the$\pi^+$ intensity on the target thickness and material. The dependence of the pion production rate on the secondary beam momentum (selected by choosing the proper B$\rho$ value) is shown in fig. 3. The $\pi^-$ production rate was measured by switching the polarity of the magnets. Similar behaviour was observed in both cases. The maximum pion beam intensity was observed at about 1 GeV/c beam momentum.

The maximum pion count rate at the secondary reaction target position was achieved when the big HODOSCOPE detector was removed from the beam. It amounted to $\sim$ 200 counts/spill. The presence of the HODOSCOPE reduced the beam intensity by about a factor of 2. In the final experiment it is planned to install a thinner detector (0.3 - 0.5 cm instead of 1 cm) and replace $\sim$ 3 m of air in the beam line by vacuum. This will more or less correspond to the situation without HODOSCOPE. The increase of the primary beam intensity to its maximum value of about 10$^{12}$ particle/spill (compared to 10$^9$ particle/spill for the present run) give another improvement by three orders of magnitude. At least a factor of five is expected from changing the primary beam to proton instead of carbon and increasing the beam energy up to 3.8 GeV.

Figure 3 : The $\pi^{\pm}$ and proton intensity per spill as a function of momentum at place of reaction target.

An attempt was made to measure the beam profile at the target position by installing a small 1x2 cm$^2$ scintillator in addition to the big one (19x20 cm$^2$). The relative count rate of the small detector compared to the big one is plotted as a function of X and Y position of the small detector in fig. 4. The position was measured relative to the center of the big detector which was aligned to above 1 cm with respect to the beam axis. A proper hardware gate was imposed on the timing signal (corresponds to the TOF spectrum) to separate pions from protons.

Figure 4 : Pion beam profile at the reaction target.

To study the influence of the beam on the performance of a gamma detector we placed a germanium detector close to the reaction target position at about 10 cm from the beam spot. Due to mechanical limitations it was not possible to move the detector closer. No strong $\gamma$-ray or charge particle signals were observed in the germanium detector due to beam related activity.
 

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