SIDDHARTA (SIlicon Drift Detector for Hadronic Atom Research by Timing Application) aims for a precise determination of the shift and width of kaonic hydrogen at the percent level. To achieve this goal large area detectors (SDDs – Silicon Drift Detector) have been developed within a European Joint Research Project (FP6 – Hadron Physics). A first measurement with a prototype setup of the “new developed” SDD detector system at the DAΦNE e+e– collider was started spring 2008 with the aim to study this new detector system in the environment of DAΦNE and to reduce the machine background by timing applications. The main background source in DAΦNE is originated from e+e– lost from the beam pipes, the Touschek effect and the interaction with the residual gas. The timing of these background particles is not synchronized to the timing of K+K– pair productions. Thus, using the time information on the K+K– pair production together with the SDD time information a significant background rejection can be achieved.

Before starting the kaonic hydrogen measurements, a test of the SDDs using a nitrogen gas target was performed from March to May 2008 (DAY-1 measurement). In particular, in this DAY-1 measurement, the background reduction with the triple coincidence of the K⁺, K^– and SDD timing was examined. Tests of the background reduction are very important inputs, since for the first time SDDs are used in an accelerator environment. The information of the background reduction and of the condition of the beam background is crucial for the success of the kaonic hydrogen X-ray measurements. The readout system for the DAY-1 setup was specially developed and designed for this DAY-1 experiment by SMI. Fig. 1 shows the SDD energy spectra with and without triple coincidence. The top figure shows the energy spectrum without coincidence. Cu and Mn X-ray peaks are clearly seen, which were produced from a Cu foil and an Fe-55 source to provide the calibration lines. The bottom shows the energy spectrum using the triple coincidence. The kaonic nitrogen peaks at 4.6, 7.6 and 14 keV are clearly seen with a S/N ratio of about 1:1. In addition, the Cu and Mn calibration lines are seen because of the accidental coincidence events. Comparing the intensities of the calibration lines between with and without the coincidence, a background rejection capability of more than 10⁴ was obtained.

During August and September 2008 the final setup was moved to DAΦNE and installed at the interaction region. Fig. 2: Day-1 setup showing the SDDs and the cryogenic target system. Fig. 3: Cryogenic target cell surrounded by SDDs.
The final system consists of a cryogenic target cell surrounded by SDDs with an active area of total 144 cm² (see Fig. 3). The target cell and the SDDs are working at low temperatures: 25 K and 170 K, respectively. Therefore, the whole setup has to be placed inside a vacuum chamber with an insulation vacuum below 10^-5 mbar, achieved by a wide range turbo molecular pump.

After a debugging phase a first data taking period started from October to December 2008 (but only in parasitic mode), with the main goals to optimize the kaon stops in the target cell and to proof the stability of the detector system in the harsh environment of an electron-positron collider. The target cell was filled with gaseous helium and the kaonic helium L-lines were measured (the X-ray yield in this case is at least a factor of 10 higher than for kaonic hydrogen). With this measurement we were able to proof that the developed detector system fulfils all our requirements and in addition, this measurement was so successful that the kaonic helium data will be published within the first half of 2009.

 

Outlook

A measuring period is foreseen to determine shift and width of kaonic hydrogen and deuterium from January to June 2009. The second half of the year is clearly devoted to analyze the two measured data sets of hydrogen and deuterium. In addition, we plan to ask for beam time in the second half of 2009 to perform a precision measurement of the kaonic helium isotopes.