Golubev, P.

Pietralla, N.

Reese, M.

Rudolph, D.

Golubev, Pavel

Rudolph, Dirk

Lund University Publications

Position Sensitivity of LYCCA Time-of-Flight Detectors

GSI Repository

Position Sensitivity of LYCCA Time-of-Flight Detectors∗M. Reese1, N. Pietralla1, P. Golubev2, D. Rudolph2 for the PreSPEC-AGATA-collaboration1Technische Universita¨t Darmstadt, Germany; 2Lund University, SwedenIntroductionThe Lund York Cologne CAlorimeter (LYCCA) [1, 2] isan essential system of the PreSPEC-AGATA setup [3] lo-cated at GSI. Its main purpose is the identification (Z ,A)and tracking (~v) of reaction products from the secondarytarget. The Z identification is done with a large ΔE − Etelescope, consisting of Double-sided Silicon Strip Detec-tors (DSSSD) for the energy loss as well as position mea-surements, and CsI scintillation detectors to measure theresidual particle energy. An additional time of flight (ToF)measurement is used to determine the particle mass. Thin,circular-shaped plastic scintillation detectors, read out with12 or 32 photomultiplier tubes (PMTs), are used as toachieve the required timing precision [1, 4]. The large di-ameter of 27 cm of the ToF-Start and ToF-Stop membranesnecessitates a correction for the path length of the photonsinside the membrane. Therefore, the ion position insidethe membrane has to be known and is usually determinedwith additional tracking detectors. This report describes amethod of determining the particle position, based only onthe 32 time signals from the large membrane’s PMTs.Method and ResultsThe position information is implicitly given as the min-imum of the log-likelihood function logL(~rp) of the par-ticle hit position ~rp on the membrane. Given the N = 32PMT time signals, ti, it can be written aslogL(~rp) =1NN∑i=1t2c,i (~rp) −(1NN∑i=1tc,i (~rp))2(1)where the measured time signals, corrected for the photonpath length, are defined by tc,i (~rp) = ti− ti,prop (di). Thepropagation time, ti,prop (di), is the time a photon needsto travel from the particle hit position in the membrane ~rpto position ~ri of the i-th PMT. It is assumed to be a linearfunction of the photon travel distance d i (~rp) = |~ri − ~rp|.The two linear coefficients can be determined beforehandby a calibration procedure, using known particle positionsmeasured with the DSSSD tracking and plotting the knowndistance di versus ti − 〈tc〉, as shown in fig. 1. The aver-age 〈tc〉 =∑i tc,i is the best estimate of the time whenthe particle passes the membrane. The minimum of (1) isdetermined numerically by an iterative procedure, startingat the center of the membrane. As a final step, additionalempirical corrections for small offsets and shearing effectsare applied.∗Work supported by BMBF NuSTAR.DA - TP 6, FKZ: BMBF05P12RDFN8 (TP 6) 0 100 200 300 400 500 600 700 800 50  100  150  200  250d7 [mm] 2 2.5 3 3.5 4t 7 - <t c> [ns]Figure 1: Linear correlation between tc,7 (~rp) and d7 (~rp)for the 7-th PMT.The spatial resolution can be deduced from the differ-ences between the positions given by DSSSD detectors andthe result from the membrane detector. The resulting distri-bution is shown in fig. 2 and has 7.6 mm FWHM along thex and 6.8 mm along the y-direction, respectively. Takinginto account the 3.6 mm wide DSSSD strips, the resultingposition resolution for the membrane detector is 6.7 mmand 5.1 mm FWHM in x and y-direction, respectively.The reason for the difference in x and y is not yet under-stood. The analysis procedure was done using a new anal-ysis framework for PreSPEC-AGATA data analysis [5] anddata from a 132Sn fragmentation setting. Upgrades of theLYCCA ToF system foresee the digitization of PMT sig-nal heights, which should further improve the membranes’intrinsic position resolution.-15 -10 -5  0  5  10  15counts [a.u]xm - xDSSSD [mm]Figure 2: Histogram (solid line) of x-position differencefrom membrane (xm) and DSSSD tracking (xDSSSD). Thedashed line is a Gaussian fit (FWHM = 7.6mm).References[1] D. Rudolph et al., LYCCA Technical Design Report (2008);P. Golubev et al., submitted to Nucl. Instr. Meth. A.[2] A. Wendt et al., GSI Sci. Rep. 2010, PHN-NUSTAR-FRS-03.[3] P. Boutachkov et al., this report.[4] R. Hoischen et al., Nucl. Instr. and Meth. A 654 (2011) 354.[5] M. Reese et al., this report.GSI SCIENTIFIC REPORT 2012 PHN-ENNA-EXP-55185

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Position Sensitivity of LYCCA Time-of-Flight Detectors

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