The BBS has three iron magnets that are used to bend charged particles over an angle of about 60 degrees. The instrument is designed such that an image of a target spot is produced at a so-called focal plane. At the position of this focal plane detection systems can be mounted to measure the position and the angle of a charged particle traversing the focal plane.
The original design is based on a study made by Seigo Kato (Yamagata University, Japan) and further developed at KVI. See also the documentation for more details on the development.
The spectrometer is of a QQD-type and has therefore two quadrupole magnets (Q) and one dipole magnet (D). The relative position of two quadrupoles with respect to the dipole magnet and with respect to the target (= object point) can be changed over a distance of 600 mm. Three configurations are possible; they are called mode A, B and C.
In mode A the quadrupole doublet is moved towards the target position and the solid-angle acceptance reaches 13 msr; at the same time the momentum-bite acceptance is 13% (i.e. particles with a magnetic rigidity deviating less than 6.5% from the nominal rigidity are accepted in the spectrometer). In this setting the distance T between the target position and the Effective-Field Boundary (EFB) at the entrance of the first quadrupole is 0.84 m. In mode C, where the doublet is moved away from the target towards the dipole magnet (T = 1.44 m), the situation is reversed: the solid angle decreases to 6.7 msr, and the momentum acceptance increases to 25%. Mode B, finally, is a setting intermediate to modes A and C.
The design of the entrance region of the spectrometer allows enough free space around the scattering chamber for the user to set up large detection systems for the measurement of decay products in coincidence with ejectiles measured in the focal-plane detection system (e.g. for decay or fragmentation studies). In addition, the magnets are designed such that the high-momentum side of the spectrometer can be used either as a beam-dump area or to allow a free passage for the direct beam from the cyclotron into an well-shielded external beam dump. These design features and the three different operation modes make this spectrometer a unique instrument that can be used for many different experiments in nuclear physics.
The aberrations due to either the size of the solid angle or to the momentum acceptance will be reduced using ray-tracing methods. For this purpose the vector that describes the trajectory of a particle at the point of impact in the focal plane, will be determined using state-of-the-art focal-plane detection systems. Event-by-event corrections will be employed to reduce these aberrations.
In an attempt to minimize the corrections needed, higher-order components have been added to the dominant components of the two quadrupole magnets and of the dipole magnet. The parameters used in this optimization procedure are:
- for the two quadrupole magnets the ratios of the sextupole-to-quadrupole strength and the octupole-to-quadrupole strength (S/Q and O/Q, respectively);
- for the dipole magnet the second-order and third-order coefficients describing the curvature of the EFB at the entrance (S02 and S03) and at the exit (S12 and S13).
The design was optimized for the three different modes A, B and C at the same time. Mode B has relatively small aberrations, whereas in the other two modes the aberrations mentioned increase. Some of these aberrations remain non-negligible, such as (x|theta*delta) which influences the value of the focal-plane tilt angle. In the design this angle is 54 degrees in all three settings, which can be a drawback if one wants to mount a focal-plane detector along the focal plane itself.
The magnetic configuration is of a QQD type with sliding quadrupole magnets. This combines a large solid angle with a small momentum bite and vice versa. Therefore, by sliding the quadrupole magnets both the opening angle (horizontal and vertical acceptance) and the momentum acceptance will change.
Going from one extreme setting of the quadrupole doublet to the other, the solid angle changes from 6.7 to 13.0 msr. The horizontal angular acceptance changes from 60 to 72 mrad (3.4 to 4.1 degrees) and the vertical angular acceptance changes from 112 to 180 mrad (6.4 to 10.3 degrees). At the same time the momentum bite acceptance changes from 25 to 13% (see the Table).
With a nominal bending radius of 2.2 m and a maximum dipole field strength B = 1.4 T, the maximum nominal rigidity is 3.08 Tm, corresponding to a maximum nominal momentum of 921 MeV/c for electrons or protons. The deflection angle is 60 degrees.
The focussing properties are point-to-point both in the horizontal (dispersive) plane and the vertical plane. The first-order energy resolving power (ERP) is 2500 and is determined by:
the lateral dispersion (2.57 m),
the horizontal magnification (0.45), and
the lateral dimension of the object point (1 mm).
The correction for intrinsic and kinematical aberrations will be done by software (raytracing procedures).
The ion-optical design has been made using the code Raytrace with a simultaneous optimization for mode
|Laatst gewijzigd:||28 januari 2014 12:04|