Introduction to GEAMAG

Introduction

It was first proposed in 1996 [1] to construct a large-acceptance magnetic spectrometer MAGNEX at the LNS, Catania, for the future radioactive beam facility EXCYT [2]. As shown in the MAGNEX report, because of the large acceptance of the spectrometer a strong effect of the spherical and chromatic aberrations was foreseen. To compensate for these aberrations, a detailed determination of the magnetic field and a careful choice of the detector shape and performance are fundamental. In an initial study [3], the magnetic field and the detector configuration were considered separately. It was then decided to make a detailed simulation of the combined whole spectrometer using the computer tool GEANT [4].

The principal studies undertaken so far with GEAMAG are:

Momentum (energy) resolution for different particle types, targets, gas and intrinsic detector resolutions.
Effect on resolution of:
- putting detector at an angle to the focal plane,
- fluctuations in magnetic field of dipole [5],
- kinematic factor.
Efficiency of the spectrometer for different sets of window support grids, field wires and detector shapes.
Efficiency of silicon array.
DE (Z) resolution corrected for q_foc.
Time-of-flight (Mass) resolution.
Detector low energy threshold.
Illustration of some potential experiments:
- ⁶Li(¹⁴O,¹²C)⁸B
- ¹⁴C(⁷Li,⁷Be)¹⁴B
- ¹²C(⁶Li,¹²C a)d
Ionic charge states.
PSD, target chamber geometry.

The beams from the EXCYT facility will be light to medium mass (A < 40) with typical tandem van der Graaf energies. The table below gives the parameters for some particles studied with GEAMAG.

List of particles used in GEAMAG simulations. The central momentum (energy) is given. In the simulations, particles at ± 5% and ± 10% of this central momentum may be generated, and a small random spreading introduced. For the heavier ions, the charge state calculated by GEANT may be non-integer.
Particle	p (GeV/c)	E (MeV)	B_dipole (kG)	B_quad/r (kG.m^-1)	Q
¹H	0.240	30	5.000	-0.2340	1
⁷Li	0.800	49	5.559	-0.2602	3
¹⁶O	1.471	80	4.030	-0.1886	7.7
²³F	1.183	33	3.500	-0.1638	7.3
⁴⁰Ca	3.535	166	4.612	-0.2159	15.9

MAGNEX spectrometer overview

The design of the MAGNEX spectrometer is based on a vertically-focussing quadrupole in front of a single dipole magnet (bend angle 55^o). The second- and higher-order optical aberrations are partially compensated for by the shape of the entrance and exit field boundaries and by trajectory reconstruction (software corrections). The latter involve the use of the measured incident angles in a position-sensitive start detector (PSD) as well as the angles and positions of the particles in the focal plane detector. Such software corrections can, of course, be included in the GEANT simulation.

One of the challenges for the design of the detector is the large angle of inclination of the focal plane (-61^o). Furthermore, the length (approx 90 cm) and height (20 cm) of the focal plane are rather large. The proposed solution to this [3] is a trapezoidal-shaped box containing two high-resolution position-measuring counters separated by a traditional ionisation chamber. The focal plane will lie along the plane of the first position counter in order to reduce ballistic errors. Behind the second position counter is a wall of silicon detectors to measure the residual energy of the particles.

Required resolutions

According to the MAGNEX report [3], to achieve the desired 1/2000 energy resolution, the determination of the position in the focal plane, x_foc, should have a accuracy of ± 1 mm. The maximum allowable errors in the measurements of q_foc and f_ini are about ± 3.2 mr and ± 4.9 mr, respectively.

Bibliography

[1] A. Cunsolo et al., Proc. intl. workshop "Giornata EXCYT", Catania, Italy (1996), p. 143; Proc. intl. conf. on large-scale collective motion of atomic nuclei, Brolo, Messina, Italy, 1996 (ed. G. Giardina, G. Fazio and M. Lattuada, World Scientific, Singapore, 1997) p. 83.

[2] G. Ciavola et al., Nucl. Phys. A616 (1997) 69c.

[3] A. Cunsolo, M. Aliotta, A. Bonaccorso, F. Cappuzzello, E. Costanzo, D. Ficarra, A. Foti, M. Lattuada, M. Re, S. Romano, V. Shchepunov, C. Spitaleri, A. Tumino, D. Vinciguerra, S. Cherubini, P. Roussel-Chomaz, W. Mittig, O. Malishev, A. Popeko, H. Savajols, C. Stephan, L. Tassan-Got and A Yeremin, MAGNEX: a large acceptance magnetic spectrometer for EXCYT, INFN-LNS report, 1998.

[4] GEANT: Detector description and simulation tool (version 3.21), CERN Program Library Long Writeup W5013, Applications Software Group, CERN, Geneva, Switzerland (1997).

[5] C. Nociforo, tesi di dottorato, University of Catania, (2001).

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Last Modified: 16 March 2002