BigBite

P.O. Box 41882
1009 DB Amsterdam
The Netherlands

BigBite: A large-acceptance electron spectrometer for the internal target facility at NIKHEF

D. de Lange, R. Arink, C.W. de Jager, J.J.M.Steijger, H. de Vries (NIKHEF)
M. Anghinolfi, M. Ripani, M. Taiuti (INFN, Genova, Italy)
D. W. Higinbotham, B. E. Norum (UVa, Charlottesville, U.S.A.)
S. Popov, E. Konstantinov (Budker Institute, Novosibirsk, Russia)

Introduction

The first experiment in the Internal Target Facility at NIKHEF on tensor-polarized deuterium (91-12) has indicated the powerful capabilities of this setup. in the 91-12 experiment a non-magnetic electron calorimeter was used. It has a verly large acceptance but a momentum resolution of 10%. For future experiments a new electron detector is needed with a better momentum resolution. Therefore a new spectrometer, called BigBite, has been designed and constructed. It consists of a non-focusing magnet and features track reconstruction.

ITF experimental parameters

Detection System

The detection system consists of:

Two sets of wirechambers
A plastic scintillator
An aerogel Cherenkov detector

Wirechambers

The track reconstruction will be done with two sets of wirechambers. The two coordinates in the wirechambers are obtained by different methods:

in the dispersive direction:
Drifttime measurements using wire signals
in the non-dispersive direction:
Charge distribution using cathode-strip signals

Properties wirechambers

Wire pitch	20 mm
Strip pitch	5.08 mm (0.2")
Strip width	4.08 mm
Wire resolution	100 - 200 µm
Strip resolution	80 -150 µm
Active area	35 x 140 cm^2 50 x 200 cm^2

Scintillator

The scintillator will provide the trigger for the electronics and a reference time for the driftchambers. It consists of a 1 cm thick slab of plastic scintillating material (NE-110A) with an active area of 200 x 50 cm^2;. The scintillator is read out by four photomultipliers, two at each end.

Cherenkov

An aerogel Cherenkov detector will be used as an electron-pion discriminator. The Cherenkov has an active area of 210 x 50 cm^2; and a height of 24 cm. The Cherenkov is filled with 9 cm aerogel with a refractive index of 1.05. Light conversion is performed by twelve photomultiplier tubes.

Magnet

BigBite consists of one dipole. This dipole has the following properties:

Central deflection angle	25°
Gap of the magnet	25 cm
Horizontal acceptance	±80 mrad
Vertical acceptance	±300 mrad
Maximum magnetic field	1.2 Tesla

At the maximum field settings electrons are accepted with a momentum between 250 and 900 MeV/c.

Support System

There is one support for the magnet, the detector and the read-out electronics. This support can be rotated in the range of 25°-90° with the use of airpads.

Design Properties BigBite

Momentum resolution	0.5 %
Angular resolution	3 mrad
Momentum acceptance	± 50 %
Solid Angle	96 msr
Extended target capacity	>= ± 10 cm
Out of plane acceptance larger than in plane

Resolution

Monte Carlo studies show that multiple scattering and track resolution contribute equally to the momentum and angular resolutions of BigBite. These simulations predict:

momentum resolution	0.5 %
angular resolution	3 mrad