Frühauf, J.

Gerbig, J.

Heil, M.

Hoffmann, J.

Kelic-Heil, A.

Koch, K.

Kurz, N.

GSI Repository

A new Time-of-flight wall for R3B ∗M. Heil1, A. Kelic´-Heil1, J. Gerbig2, J. Fru¨hauf1, J. Hoffmann1, K. Koch1, N. Kurz1, for the R3Bcollaboration1, and the FAIR@GSI RBRB11GSI, Darmstadt, Germany; 2Goethe-Univerita¨t, Frankfurt, GermanyIntroduction and design goalsThe beams provided by Super-FRS at FAIR will havehigher energies and will be more intense than presentlyavailable. The goal of the new R3B setup is to fully ex-ploit the potential of FAIR beams. With the new superconducting magnet GLAD it will be possible to deflectfully-stripped ions up to the Pb region with energies up to1 AGeV. Also the detectors of the R3B setup should be ableto scope with the new conditions.One detector of the setup is the time-of-flight (ToF) wallmade of plastic scintillators. The purpose of this detectoris the measurement of the time-of-flight and the nuclearcharge of the fragments after the reaction in the target. To-gether with the measured trajectory through a dipole field,the mass of the fragment can be identified. The charge isobtained by precise energy loss measurements of the frag-ments passing through the scintillators. With the Super-FRS at FAIR it will be possible to accelerate fully strippedbeams up to the Pb region and consequently, the new ToFwall should be able to separate charge Z from Z-1 evenfor heavy fragments. For Pb fragments the energy lossmeasurements for Z and Z-1 are seperated by 2.4 % and acharge resolution of σΔE<1% is needed in order to resolvethem. Furthermore, since the unreacted beam also hits thefragment wall, the detector must be able to maintain theperformance even at high beam rates up to 1 MHz. In or-der to match the momentum resolution of other parts of thesetup the relative time-of-flight resolution should be around0.1‰. Since the detector is placed typically between 12 and20 m behind the target the shortest expected flight times areabout 75 ns. This results in a required time resolution ofσt<17 ps (see also contribution [1] in this annual report).Although the current version of the ToF wall was suc-cessfully used in many experiments in the past, we plan tobuild a new ToF detector with superior time and energy res-olution at beam rates up to 1 MHz. It is not planend to use acompletely new detector concept but rather take advantageof the experience collected over the last years and advancethe current design.The detector will have four planes of scintillators and theactive part will cover an area of 120x80cm2. Each planeconsists of 43 scintillators with the size 800x27 mm2 andthey are read out with photomultipliers on both far ends.The first two planes will have scintillators with a thicknessof 3 mm and the last two planes a thickness of 5 mm, re-spectively. In this contribution we will focus on the perfor-mance of prototype detectors concerning the energy reso-∗Work supported by FAIR@GSI PSP code: 1.2.5.1.2.1.lution and new readout-electronics at high rates.Prototype results and developmentsIn order to test the behaviour of the detector withoutbeam, a test stand with a fast LED was developed (see mas-ter thesis of Julian Gerbig [2]). The LED emits light in thesame wavelength region as the BC408 plastic scintillator,and the intensity as well as the pulse rate can be varied. Inorder to obtain realistic conditions, the LED was pulsed inrandom mode.Figure 1: The plot on the top shows the relative chargeresolution for each frequency for two differnet PMs. Thebottom plot shows the shift in the charge measurments fordifferent rates.Two different PMs from Hamamatsu were tested, bothwith a diameter of 1 inch. Model H6533, a very fast PM(TTS: 0.25 ns) but without active voltage divider. ThisFG-S-FRS-04 GSI SCIENTIFIC REPORT 2013336 doi:10.15120/GR-2014-1-FG-S-FRS-04model was used in the past experiments for the existingToF wall (NTF). The second model R8619 is a cost effec-tive PM with an active voltage divider (TTS: 1.2 ns).At the beginning, the conditions of one of the last ex-periments were simulated. The intensity of the LED wasset to simulate medium heavy nuclei, such as e.g. Ni andthe voltages of the PMs were set in order to extract largecharges as needed for the previously used read-out. Thefrequency was varied between 5 and 800 kHz. Althoughthe energy resolution for both PMs was good for each in-dividual rate, it was observed that the peak position shiftedwith frequency (see figure 1). In an experiment with vary-ing beam rate during a spill this would lead to a bad reso-lution. Especially for the H6533 the shift is dramatic. Forthe R8619 the shift is much smaller but still visible and toolarge for resolving charges of heavy fragments.The shift in the charge measurement stems from thePMs. Especially for large currents, the voltages at the lastdynodes can not be kept constant and the gain of the PMis changing. The situation can be improved by taking ac-tive voltage dividers and by reducing the HV of the PMsand therefore also the anode current. But also the charge ofthe signal plays an important role. For small signal chargesthe R8619 is suited to measure the signals with a resolution(and shift) of less than 1% even for rates as high as 800 kHzas can be seen in figure 2.It can also be seen that for too small charges, the rela-tive resolution for charge measurements gets worse again.Therefore, for an excellent performance of the new ToFwall is is mandatory to use PMs with active voltage di-viders and to reduce the signal amplitudes via the HV ofthe PMs to a region were the PMs work best with regardto rate stability and charge resolution. This also requiresa change in the read-out electronics. So far, CAEN TDCwere used and the signals of the PMs were split in a timeand an energy branch. The analog signal for the energybranch was delayed by about 600 ns until the trigger decis-sion was made. Especially this long delay caused a damp-ing of the signals by a factor 10. The new read-out will con-vert the analog signals immediatelly in logic signals with asignal width which is either propotional to the time-over-threshold (ToT) or to the integrated charge (charge to timeconverter QTC). This signal is then recorded by a multi-event FPGA TDC such as the VFTX2 [3] developed at GSI.From the leading edge of the signal the timing can be ob-tained (see also contribution [4] in this annual report) andfrom the trailing edge the energy can be restored. In thisway the signal is immediatelly digitized and no delays areneeded. Since the VFTX2 is multi-hit cabable the hits canbe read out much later.Summary and outlookentsIt is planned to build a new ToF wall for the R3B setupat FAIR. As the existing one, it will consist of plasic scin-tillators but we aim for an improved energy (σΔE<1 %)and time resolution (σt<17 ps) even at higher beam rates.Figure 2: Shown are the charge resolution and the shift inmean energy for the PM R8619 for different loads of thePM. It can be seen that for medium charges (260 pC) asufficiently good resolution can achieved, even at rates upto 800 kHz.A test setup with a LED as light source has shown that thisis possible with the use of PMs with active voltage dividersand with an adapted electronic read-out which allows to ex-tract only small charges from the PMs. The electronics willbe developped by the GSI EE department. We plan to provethe performance of a prototype in beam tests and detail thefinal layout of the new detector in a TDR.References[1] A. Kelic´-Heil und M. Heil, “New time-of-flight system forthe R3B set-up”, contribution to this annual report.[2] J. Gerbig, “Kalibrierung eines Szintillationsdetektors mitHilfe von LEDs”, Masterarbeit 2013, Goethe-Universita¨tFrankfurt[3] J. Fru¨hauf, J. Hoffmann, E. Bayer et al., “VFTX (VME-FPGA-TDC 10ps)”, Scientific Report 2012 GSI Report 2013-1, 300 p. (2013)[4] R. Plag, M. Gilbert, M. Heil et al., “High precision multi-hit time-of-flight measurements at R3B”, contribution to thisannual report.GSI SCIENTIFIC REPORT 2013 FG-S-FRS-04doi:10.15120/GR-2014-1-FG-S-FRS-04 337

A new Time-of-flight wall for $R^3B$

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A new Time-of-flight wall for $R^3B$

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A new Time-of-flight wall for R3BR^3BR3B

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A new Time-of-flight wall for $R^3B$