Ackermann, D.

Geissel, H.

Nociforo, C.

Weick, H.

Winfield, J. S.

Winkler, M.

Yavor, M. I.

GSI Repository

Mass measurement by track reconstruction with the LEB spectrometerJ.S. Winfield∗1, H. Geissel1,2, D. Ackermann1, C. Nociforo1, H. Weick1, M. Winkler1, and M.I. Yavor31GSI, Darmstadt, Germany; 2JLU, Giessen, Germany; 3RAS, St. Petersburg, RussiaThe large-acceptance spectrometer in the LEB cave willbe used for experiments with intermediate or low energy ra-dioactive beams from the Super-FRS. Gamma and charged-particle spectroscopy will be performed around a typicallylow-mass secondary target. Forward-focussed fragmentsare transported to the spectrometer’s focal plane, wherethey are identified or “tagged” to complete the reactionanalysis. The large beam emittance at the target means thatevent-by-event tracking is essential (position, angle, time-of-flight as well as the magnetic rigidity in the Super-FRS).The simulations discussed here involve fragments in themass 200 region, which are assumed to come from reac-tions at about 180 MeV/u. The goal is to resolve the differ-ent neighbouring-mass isotopes.The Monte Carlo program MOCADI [1] was used forthe simulations in which the secondary beam starts fromthe end focal plane of the Super-FRS. The secondary reac-tion target is a 500 mg/cm2 beryllium foil, followed by a300 μm silicon double-sided strip detector (DSSD, P2) forthe beam spot correction and a 1-mm thick plastic scintil-lator. A TPC tracking detector P1, is located 0.5 m beforethe target. P1 and P2 would be used to correct the fragmentangle during the data analysis. However, only the uncor-rected angle is needed for the mass reconstruction. This isobtained event-by-event from P2 and P3, another positiondetector 30 cm downstream from the targetThe Lund-York-Cologne calorimeter LYCCA [2] is usedfor the measurements at the focal plane. This consistsof a 2-mm thick plastic (stop ToF), a 0.3-mm Si DSSD(ΔE, x, y: P4) and a 2.5-cm CsI(Tl) crystal (Eres).Theachieved resolutions during the commissioning of Pre-SPEC [3] are σx, σy = 0.5 mm, ΔZ = 1/100, ΔE =1/100 and ToF = 27 ps.The following reconstruction equations are used:• δ is calculated fromδ = [xf − R11xi − R12ai]/R16. (1)where xi is taken from P2, ai is calculated from P2and P3, and xf is taken from P4.• The corrected Bρ is calculated as Bρn = Bρ0(1+δ),where Bρ0 is the magnetic rigidity of the referenceparticle, 201Pt78+, before the spectrometer.• The path length of the reference particle is correctedfor the initial coordinates byln = l0 +R51xi +R52ai +R56δwhere the subscript 5 is the length parameter.∗ j.winfield@gsi.de• β is calculated from the corrected path length:β = ln/c(t2 − t1)• Finally, the mass A divided by charge Q is:AQ=Bρnγβcmu(2)where mu = 931.494061 MeV/c2.A similar reconstruction scheme is made at BigRIPS [4].The exact matrix elements are known from the ion-optics,but rather than putting these back into the equations, “em-pirical” first order matrix elements are used from a sim-ulated test experiment with a well-focused primary beam.As seen in the cross-hatched peak for 201Pt in Fig. 1, one050010001500200025002.53 2.54 2.55 2.56 2.57 2.58A/QCounts201Pt200Pt199PtFigure 1: Simulated mass reconstruction for A ∼ 200.Cross-hatched histogram is for 1st-order matrix elementsonly. The finite resolution of the detectors is included.could obtain reasonably resolved masses up to A ∼ 200;however, there is a tail on the peak at the low A/Q side. Theresolution can be improved by the inclusion of selected 2ndand 3rd order matrix elements, with appropriate modifica-tion of Eq. (1). Following the procedure used at BigRIPS,the Newton-Raphson iterative method is used to solve fordelta. There is a clear improvement in the shape and widthof the (unshaded) peaks of of Fig. 1 compared with the1st-order calculation. The FWHM of the 201Pt peak corre-sponds to 0.53 mass units.References[1] N. Iwasa et al., Nucl. Instr. Meth. B 126 (1997) 284.[2] http://www.nuclear.lu.se/english/research/basic-nuclear-physics/nustar/lycca D. Rudolf, Lund Univ., Sweden.[3] H.J. Wollersheim, Acta Phys. Pol. B 42 (2011) 705.[4] N. Fukuda, talk at Expert Meeting on In-flight Separators andRelated Issues, Dec. 2010, GSI.GSI SCIENTIFIC REPORT 2012 PHN-ENNA-EXP-75209

Mass measurement by track reconstruction with the LEB spectrometer

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Mass measurement by track reconstruction with the LEB spectrometer

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