N U C L E A R I N S T R U M E N T S AND METHODS 148 (1978)
503-509 ; ©
N O R T H - H O L L A N D P U B L I S H I N G CO.
A Novel Position Sensitive Detector for Photons and Electrons
A NOVEL ZERO TIME DETECTOR FOR HEAVY ION SPECTROSCOPY*
J. DAVID BOWMAN and R. H. HEFFNER
K. W. Brown*, Z. Q. Gosser†, J. M. Krupa*, A. Liao*, J. P. Smith*, T. K. Steinbach†, S. Hudan, R.T. de Souza
University of California, Los Alamos Scientific Laboratory, Los Alamos, NM 87545, U.S.A.
Received 5 July 1977
Indiana
University, Department of Chemistry and Center for Exploration of Energy and Matter
Nuclear!
Chemistry!
The design, construction and test of a heavy ion time pick-off system are described. The detector presents a homogeneous
foil of a few/zg/cm 2 thickness to the beam. Secondary emission electrons are isochronously transported to a channel plate
electron multiplier by crossed electric and magnetic fields. A time resolution of (230+30) ps has been measured using
natural alpha particles and a time coincidence between a surface barrier silicon detector and the foil time pick-off system.
Sources of the measured time dispersion are discussed.
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1. Introduction
tector could also be used to derive a zero time sigor leaving a magnetic
The detection
and identification of low energy nal for particles
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spectrometer.
heavy ions represent a special experimental chalThe conceptual design of the apparatus is dislenge due to their short range and susceptibility to
multiple scattering. For example, a 11B ion with a cussed in section 2. In section 3 we describe our
momentum of 250 MeV/c, which is typical of mo- design studies and in sections 4 and 5 we describe
mentum transfers in nuclear reactions, has an en- the construction and experimental tests performed
ergy of 5.5 MeV. Such an ion would be scattered with the apparatus.
through an average angle of 60 mr upon passing
2. Conceptual design
through 1 mg/cm 2 of silicon and would be
In addition to the requirements of simplicity of
stopped by 1.7 mg/cm 2 of silicon. Thus the use of
even the thinnest silicon detectors as time pick-off construction a good heavy ion time pick-off syselements or energy loss detectors would not be tem requires:
a) good time resolution
possible for these particles.
b) low energy loss, low multiple scattering, and
In heavy ion spectroscopy one wishes to detect
?D8".=2<"#0<#5BK&L#D%&MNK&>00,*&OK&PQQRK&!.H*0&PPSTPUV& the absence of thickness inhomogeneities,
such particles with high efficiency and to measure
c) a high, well known efficiency and
their mass, nuclear charge and energy. In general
d) a large acceptance aperture.
it is necessary to measure three properties of the
A schematic picture of our time pick-off system
detected particles; usually the total energy, rate of
is
illustrated in fig. 1. The incident ions pass
ionization and velocity or radius of curvature in a
magnetic field are measured. This article deals through thin cellulose nitrate-backed carbon foils
$ of the measurement of the ve- knocking out secondary emission electrons. These
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with the problem
electrons are transported by crossed electric and
locities of heavily ionizing particles.
Because of the difficulties discussed above, time magnetic fields to a chevron channel plate electron
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multiplier which acts as an electron detector. The
pick-off schemes
which
involve
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salient features of this detection scheme are as follow energy electrons emitted when heavy ions .<<*D*".8*)&7B&.3&*D*<8"2<&]*D)&.3)&7*38&7B&.&=.H3*E<&]*D)&#38#&89*&FA!%&
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pass through very thin carbon foils ( = 20 # g / c m 2) lOWS :
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have been developed and reportedl-S,9). We have &
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developed a foil time pick-off system which is ~ ; I-BIt ,
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struction as well as its low cost. Two such time
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pick-off detectors have been used to measure parELECTRON
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DETECTOR
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being carried out at LAMPF6). A single such deFig. 1. Schematic picture
of the time pick-off system showing
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* This work was supported by Contract #7405-ENG-36 from
the U.S. Energy Research and Development Administration
to the University of California.
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