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Subatomic Physics – K=600 Magnetic Spectrometer

The K=600 magnetic spectrometer at iThemba LABS is a high resolution kinematically corrected magnetic spectrometer for light ions. It has the capability to measure inelastically scattered particles and reactions at extreme forward angles that includes zero degrees, making it one of only two facilities worldwide (the other being at RCNP, Japan) where high energy resolution is combined with zero degree measurements at medium beam energies. The advantage of such measurements is the selectivity it provides to excitations with low angular momentum transfer.

The K=600 consists of five active elements namely a quadrupole, two dipoles and two trim coils, referred to as the K and H coils.  It is based on the design of the former K=600 magnetic spectrometer at the Indiana University Cyclotron Facility. As an indication of its size, the average flight path for a particle from target to detector is approximately 8m.   There are three different focal-planes,  which enables measurements at a momentum dispersion of either 6.2, 8.4 or 9.8 cm/%.

The focal-plane detector are positioned behind the second dipole, and consists of position sensitive multi-wire drift chambers (MWDCs) and a pair of plastic scintillation detectors. By employing dispersion matching techniques an energy resolution of 30 keV FWHM can be achieved at 200 MeV for (p,p’) reactions, exploiting the excellent resolving power (1/28000) of the spectrometer. For more details see R. Neveling et al., NIM A 654 (2011) 29.

The recent addition of coincident particle and gamma detection capabilities further enhances the selectivity of the K=600 magnetic spectrometer, and opens up a host of new opportunities to be explored.


The K=600 and CAKE

For many years the sliding seal scattering chamber donated to iThemba LABS by NIKEF was exclusively used in all experiments. The geometry of this 524mm diameter scattering chamber is however not optimized to accommodate ancillary detector systems, either arrays of double sided silicon strip detector (DSSSDs) inside or HPGe detectors outside the scattering chamber.  A new egg-shaped scattering chamber was therefore designed. This smaller scattering chamber was manufactured from three pieces of solid aluminium, and consists of a central section onto which services are connected and a removable egg-shaped hemisphere on either side for easy access to the interior of the scattering chamber for detector mounting purposes.

The Coincidence Array for K600 Experiments (CAKE) is one such detector that can be mounted inside the new scattering chamber.  CAKE consists of up to six wedge-shaped double sided silicon strip detectors (DSSSDs) of the MMM design, placed in a lampshade configuration upstream from the target ladder. It enables coincidence spectroscopy of charged-particle decays following inelastic scattering and transfer reactions detected by the focal-plane detectors of the K=600.  For more details see P. Adsley et al., JINST 12 (02), T02004 (2017).



The K=600 and BaGeL

Coincident gamma detection capability was recently added to the K=600 repertoire through the commissioning of the highly flexible and configurable BaGeL (Ball of Ge and La3Br detectors) array.  The BaGeL infrastructure can support a gamma detector array consisting of either HPGe or La3Br detectors in a clamshell configuration, with two arms focussed around the new egg-shaped scattering chamber. The center of rotation for the two arms are 828 mm from the beam center, which allows the detectors to be moved approximately 2m away from the beam-line during beam tuning activities. This ensures minimum neutron damage during beam tuning to  sensitive detectors such as the HPGe Clover detectors available at iThemba LABS.   Coincident detection techniques are nothing new, nor is it the first time that such measurements can be performed with the K=600. However, the efficiency and granularity of the new setup, and also the fact that these measurements can be made at zero degrees, all combine to make BaGeL a valuable new experimental tool.




K=600 Magnetic Spectrometer – Technical Information

Property Low Normal High
Maximum momentum per charge p/Q (MeV/c) 860 1080 1005
Maximum proton energy (MeV) 334 493 437
Maximum magnetic rigidity (T-m) 3.00 3.60 3.50
Maximum dipole fields, D1/D2 (T) 1.23/1.64 1.64/1.64 1.64/1.23
Nominal bend radius (m) 2.1 2.1 2.1
Nominal bend angle (degrees) 115 115 115
Maximum solid angle \Delta\theta\Delta\phi (msr) 6.0 6.0 6.0
Maximum radial acceptance \Delta\theta (mrad) \pm44 \pm44 \pm44
Maximum axial acceptance \Delta\phi (mrad) \pm44 \pm44 \pm44
Momentum range p_{max}/p_{min} 1.131 1.097 1.063
p/\deltap: Resolving power \sim? \sim28000 \sim?
(x|\frac{\Delta p}{p}): Momentum dispersion (cm/\%) 6.2 8.4 9.8
Energy dispersion (keV/mm) (for 200MeV protons) 52? 42 31
(x \mid x): Horizontal magnification at p_{max} -0.42 -0.54 -0.68
(y \mid y): Vertical magnification at p_{max} -4.0 -5.7 -11.8
(\theta \mid \theta): \sim1/(x \mid x) -1.90 (\sim 1/(x \mid x)) \sim1/(x \mid x)
(\phi \mid \phi): \sim1/(x \mid x) -0.17 (\sim 1/(y \mid y)) \sim1/(x \mid x)
Focal plane length, horizontal (cm) 81 78 62
Tilting angle of focal plane w.r.t. central ray ? 35.75^{\circ} 32.2^{\circ}
Beam dispersion (according to L.Conradie) 1/16000 1/16000 1/16000
Beam dispersion (according to L.Conradie) 16 cm/\% 16 cm/\% 16 cm/\%
Design parameters of the three dispersion modes of the K=600 spectrometer.