The goal of this experiment was to measure the charge-state distribution, energy loss, and energy-loss straggling from 500 MeV/u down to energies of 40 MeV/u. For the measurement six different target materials, Be, C, Al, Cu, Ag and Au, with different thickness each corresponding to roughly 10%, 20% and 30% energy loss were used. The energies of the primary beam delivered from the heavy ion synchrotron SIS18 was 500, 300, 100, and 50 MeV/u for the 136Xe beam and 300, 100, 70, and 50 MeV/u for the 58Ni beam.
3.1 Slowing down experiment with 58Ni and 136Xe ions
The Fragment Separator FRS as an energy-loss spectrometer is an ideal tool to measure charge state distributions, energy loss and energy-loss straggling. The following chapters will describe the method used for this experiment and the ion optical mode used for these measurements as well as the detectors and targets. The basics about the in-flight technique, fragment separators especially the FRS and the Bρ−∆E−Bρ separation method were already described in chapter 2.2.2.
3.1.1 Ion-optical mode
For the slowing down experiments the FRS was used in its standard achromatic mode [GEI92] as shown in fig. 3-1. As only primary beam was used there was no production target at TA, but a stripper to get fully stripped incident ions.
+20
-20 0.0m
10.0 m
71.8 m
TA F1 F2 F3 F4
Dispersion [cm/%]Y-max 0.2 mX-max 0.2 m
fig. 3-1 The FRS in achromatic standard ion-optical mode. The upper two plots show the beam envelope in x- and y-plane for an incident phace space of ε =20πmm⋅mrad. The lower plot shows the dispersion curve for this standard achromatic mode.
F1 F2 F3 F4
MWPC11
MWPC21/22
MWPC31
MWPC41 target
beam
fig. 3-2 Positions of the Multi wire proportional chambers (MWPC) for the charge-state and energy-loss measurements. For the charge-state distribution measurements MWPC31 was used. The energy loss for the materials placed at F2 was determined at F3 and F4 using MWPC31 and MWPC41 respectively.
The different targets were placed at the second focus F2 behind a collimator as shown in fig. 3-1. The positions of the Multi Wire Proportional Chambers (MWPC) described in chapter 3.1.2.2 to measure the charge-state distributions and energy loss data are shown in fig. 3-2. The charge state distribution measurements were done using the MWPC31 at the third focus F3. The energy loss measurements were done at F3 and F4 by also using these MWPCs. All energy straggling data were taken with the MWPC41 in an optics mode with a focal length of only 590 mm behind the last quadrupole to avoid the additional contribution to the peak width from angular scattering in the exit window. The overview of the detector setup behind the last Multi wire as it was used for all range focusing experiments is shown in fig. 3-3.
MWPC 41 Music 1
Slits Wedge Degrader
Music 2 MWPC 42 SCI 42
Disc Degrader
FRS
x z y
FRS -window
fig. 3-3 Detector setup at the end of the FRS. The detectors mounted are MUSICs, MWPCs and scintillators. For the slowing down experiments the beam focus was on the MW41 just 590 mm behind the last quadrupole. For the range focusing experiment the focus was on the disc degrader mounted between the two MUSICs.
3.1.2 Detectors
Three types of detectors are in standard use for experiments at the FRS. These are the Multi Wire Proportional Chamber (MWPC) mounted on mechanical feed through to move them in and out of the beam at each focal plane of the FRS, the scintillators for Time Of Flight (TOF) measurements and Multi Sampling Ionization Chambers (MUSIC). They are commonly used to identify the projectiles by their energy loss.
3.1.2.1 Multi sampling ionization chamber MUSIC
The MUltiple Sampling Ionization Chamber (MUSIC) [PFÜ94] is an ionization chamber filled with P10 gas (90% Ar, 10% CH4) at about room temperature and normal pressure.
The entrance and exit windows are made from 25 µm Kapton (C22-H10-O5-N2)n with a density of 1.4 g/cm² coated with 40 µg/cm² Al. The diameter of the windows is 450 mm.
Depending on the high voltage applied the drift velocities of the electrons reach about 5 cm/µs.
When an ionizing particle penetrates through the gas, a cloud of electrons and ions is generated and by means of an applied electric field the charged particles drift towards the cathode (positive ions) and to the six-fold segmented anode (electrons). Using charge-sensitive preamplifiers, the charge of the electrons arriving at each anode is converted into a voltage which is proportional to the number of electrons. Since this number is roughly proportional to the square of the charge of the penetrating particle, the output voltage of the preamplifier is a measure for the atomic number of this particle. The preamplifier output signal is further increased and shaped by a main amplifier and digitized by an ADC and further on handled by the data-acquisition system. From the six available anodes only the signals of the middle four anodes are used. The first and the last anode serve for homogeneity of the electric field and are only connected to the high voltage.
35 60 2 100 2 100 2 100 2 100 2 60 35
Anode 1 Anode 2 Anode 3 Anode 4
Correction Anode
Correction Anode
600 mm
299
e
-+ ions
P10 gas
fig. 3-4 Scheme of the multiple sampling ionization chamber (MUSIC). All measures are in mm.
Using an additional, fast detector as a start trigger for a TAC or a TDC, the drift time of the electron cloud provides information on the x-position of the passing particle. The stop signal for the TAC or for the TDC can be derived from the output of the preamplifiers, shaped accordingly with a timing-filter amplifier.
3.1.2.2 Multi wire proportional chambers
The multi wire proportional chambers MWPCs [STE91] are well suited for a wide range of ions to measure their positions. At the FRS about 10 of this type of detectors are routinely used for beam tracking.
A schematic layout of a MWPC is shown in fig. 3-5. The plane labeled A is the anode plane, consisting of 20 µm gold-plated tungsten wires with a distance of 2 mm. The cathode X- and Y-plane are made of 50 µm gold-plated tungsten wires with a distance of 1 mm. The wire direction of the two cathodes X and Y are orthogonal to each other while the
anode wires are diagonal in a 45° angle. The spacing between the cathodes and the anode is 5 mm respectively. A pre-gap can be used to increase the amplification for low charged ions, but was not used in the present experiment.