Technical Design Report Super Fragment Separator

DRAFT
possibility to perform a current measurement here, which becomes favourable due to the uniformity
of the pulse heights.
2.4.6.5 In-flight fragment identification (Bρ, ∆E, ToF)
Magnetic rigidity measurement
The magnetic rigidity can be deduced from a position measurement in the dispersive planes, two
scenarios are considered: (a) determination during setup (b) continuous monitoring. Option (a)
requires a good knowledge of the relative magnetic field as a pilot beam is centred with known
Bρ, and the measured B-field can be calibrated using an effective radius for the dipole magnets.
Unknown magnetic rigidities can thus be determined. The centring process can be done at reduced
rate using the standard detectors (see sections 2.4.6.2 and 2.4.6.3). Option (b) is more demanding
but also required by several NUSTAR experiments (e.g. the R³B collaboration). The idea is here to
gather additional information on the incoming beam by measuring its displacement in the dispersive
plane (MF2) of the separator event-by-event. This method [35] has been successfully applied
at the FRS to improve the mass resolution in the ALADIN-LAND reaction setup for fission
fragments that were transferred to Cave B, by using a scintillator strip array. The granularity of the
strip setup determines the maximum rate that can be dealt with typically a few MHz/strip. Instead
of this conservative approach the use of PC-CVC-DD would be advantageous.
For this latter approach, a rate capability of 100 MHz/mm could be reached with a 300-500 µm
thick detector array, polished to a roughness that is below 10 µm – which is consistent with the 3 %
accuracy specified by the manufacturers – to avoid a degradation of resolution due to a lack of
detector homogeneity. Precise water-dive measurements performed in early PC-CVD-DD research
show that even PC-CVD as grown diamond material is tight, meaning without serious amount of
pinholes: no water absorption has been measured [36]. This is an indication that it is valid to assume
roughly the diamond mass density to 3.5 g/cm 3 over the whole bulk. However, the much
higher concentration of grain boundaries in the nucleation side of the film may lead to residual
graphite material (ρgraphite ≈ 2.2 g/cm 3 ) on this side which can reduce the average density. R&D
work is underway at the FRS to ensure quality of the PC-CVD material. Details for the diamond
detector setup can be found below. The online monitoring of the stability of the separator settings
will be used for feedback loops to perform e.g. automatic centring through the setup. This requires
a combined analysis and simulation framework in order to be able to determine the offsets and
compute the corrections. Here accelerator controls and Super-FRS instrumentation have to be
coupled in the most efficient way.
Specific energy loss measurement
For the required charge resolution via the specific energy loss we consider MUSIC (multiple
sampling ionization chamber [37] detectors as the optimal choice. These detectors can currently
measure rates up to 200 kHz. There are developments underway to increase these rates up to a few
MHz [38]. These detectors will be positioned at the foci MF(2, 4, 7, 10) with an active area of (40
x 8) cm², and be operated continuously at the final focal planes to identify fragments during experiments.
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