The scintillating optical fiber isotope experiment (SOFIE) is a Cerenkov-dE/dx-Range experiment which was developed initially for balloon flight to study the isotopic composition of cosmic rays in the iron region. The electronic range and hodoscope detectors use scintillating optical fibers to image the tracks of stopping charged particles and to determine their trajectory. The particle range is determined and used together with a Cerenkov measurement to determine the mass of the stopping particle. Preliminary results of a Bevalac calibration performed in August, 1984 with a prototype of the balloon flight instrument, to study the measurement precision in range and trajectory which could be attained with this detector are described

Binns, W. R.

Connell, J. J.

Israel, M. H.

Klarmann, J.

English

NASA Technical Reports Server

272 OG 9.1-7
BEVALAC CALIBRATION OF THE SOFIE RANGE
AND HODOSCOPE DETECTORS
W. R. Binns, J. J. Connell, M. H. Israel, and J. Klarmann
Department of Physics
and the McDonnell Center for the Space Sciences,
Washington University, St.Louis, MO 63130
1. Introduction. The Scintillating Optical Fiber Isotope Experiment (SOFIE) is a
Cherenkov-dE/dx-Range experiment being developed initially for balloon flight to study
the isotopic composition of cosmic rays in the iron region. The electronic range and
hodoscope detectors in this experiment use scintillating optical fibers to image the
tracks of stopping charged particles and to determine their trajectory. From this
information the particle range can be determined and used together with a Cherenkov
measurement to determine the mass of the stopping particle. This paper describes
preliminary results of a Bevalac calibration performed in August, 1984 with a prototype
of the balloon flight instrument, for the purpose of studying the measurement precision
in range and trajectory which could be attained with this detector.
2. Experiment. Figure 1 shows a cross-section of the instrument exposed at the
Bevalac. Incident particles pass through two thin plastic scintillator counters which
provide a coincidence signal after which a Cherenkov measurement is obtained in a 2.5
cm thick Pilot 425 counter. Particles exiting the Cherenkov counter pass through a
0.794 cm thick steel passive absorber. This thickness was chosen so that particles
stopping in the range detector would be above threshold in the Cherenkov counter.
The range detector consists of a bundle of scintillating optical fibers which are
proximity focused onto the face of a fiber optic reducer which was coupled to an image
intensified video camera system consisting of a G.E. TN-2505 camera and an ITT-4144,
dual microchannel plate image intensifier tube (Binns et al., 1983a and b). The CID
sensor in the camera is a rectangular array of 244x388 pixels, with each pixel having
dimensions 23x27 microns 2. The fiber bundle which is coupled onto this array consists
of about 7 X 104 fibers with length 30 cm and a 100 micron square cross-section joined
together into a solid rod with cross-section 2.7x2.7 cm 2. The fiber bundle was
constructed by first making a boule with plastic scintillator core material (KSTI-415)
and acrylic cladding. Fibers with 1.5 mm square cross-section were then drawn and 225
of these were fused into a "multifiber boule" which was then drawn again into fibers
with a 1.5 mm square cross-section. These muitifibers were then fused into solid rods
with cross-section 1.4 cm square. Four of these rods were then joined together and
coupled to the intensified camera to form the detector. When a coincidence signal
occurred, the image intensifier was gated off and a video frame was then processed
along with Cherenkov, dE/dx, and penetration counter pulse heights.
COiNCiDENCE_,.4J"" _j,_,,t INCIDENT
DETECTORS..._t.._ NUCLEUS
\ //CHERENKOV N Figure. I
STEEL _ V Experiment cross-section
_- PENETRATION
COURTER
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https://ntrs.nasa.gov/search.jsp?R=19850026620 2020-03-20T16:47:43+00:00Z
273 OG 9.1-7
3. Range Detector Results. |ton-56 nuclei stopping in the fiber bundle produced
tracks with a breadth of about 10 fibers (full width half maximum) near the end of the
particle range. The energy deposition in plastic scintillator should, however, extend
over a range of less than 1 fiber width. This track broadening is believed to be
predominately the result of coupling between fibers due to incomplete conversion of
ultraviolet photons within the fiber in which they were produced. Imperfections in the
fibers may also contribute to cross talk. Curve a) in Figure 2 is a plot of light output
from the fiber bundle versus residual particle range where the light output is obtained
by summing the light intensity along pixel columns which are nearly perpendicular to
the tracks. A running sum of three pixel columns is then taken to smooth the data.
The theoretical energy loss versus particle range is also plotted (curve b) and has been
smeared in one dimension by a gaussian with a sigma of 400 microns to simulate the
track broadening due to fiber cross talk and broadening within the intensified camera
system. These curves have been arbitrarily normalized using a "best eyes fit" so that
the energy loss and light output match for the entry end of the track. Qualitatively
these curves are similar. However in detail it is evident that there are fluctuations in
light output along the track, and these fluctuations occur roughly on the scale of a
single multifiber (about 1.5 ram). These fluctuations are believed to be the result of
nonuniformities within the fiber bundle. In addition there is evident saturation in the
scintillator output as the particle slows down and stops.
i (L I I I i
- Figure 2
Curve a) is a plot of the
light intensity from the
' fibers vs. residual range and
>" _' "% liaht intensity curve
_2!- b) is the calculated dE/dx
(Henke and Benton, 1966)
J dE/dx
I'- vs. residual range spread by
a gaussian function with
a = 400 microns.
0 All I I I I
0 0.5 1.0 1.5 2.0
RESIDUAL RANGE (g/crn z)
Figure 3 shows the distributions in range that we obtained for iron-56 nuclei with
beam energies 473 and 529 MeV/amu and having an incidence angle of l0 degrees with
respect to the normal to the fiber bundle entry side. These nuclei were selected to have
a low penetration counter signal, and they have had a first order mapping correction
applied to account for variations in the flatness of the entry window into the fiber
bundle. The range algorithm searches from the end of the track until it finds tile
maximum in light intensity as shown in figure 2. The half maximum intensity point is
then taken to be the end point of the track. From Figure 3 we obtain a sigma in the
range distributions of 260 and 280 microns for the beam energies 473 and 529 MeV/amu
respectively. (The sigma was obtained from the full width half maxima of the
distributions.) The 529 MeV/amu distribution appears to have non-gaussian wings
which we hope to reduce by further data analysis. Calculations show that the
combination of multiple coulomb scattering, range straggling, and the finite fiber size
274 OG 9.1-7
will result in a range uncertainty of about 150 microns sigma. The additional
broadening which we observe in our measurements is believed to be the result of
imperfections in our fiber bundle. We have calculated that for a range measurement
uncertainty of 300 microns and using a Cherenkov counter similar to that developed by
Webber and Kish, (1983) appropriate hodoscopes, and an Aluminum passive absorber, a
mass resolution of better than 0.25 amu can be obtained for incident angles less than 30
degrees and better than 0.30 ainu for most particles with incident angle less than 55
degrees. We expect to be able to improve the quality of our fiber bundles which should
result in improved range measurement precision.
80 473 luleV/omu 529 I_,V/om
_FL o-• _,60/zm o- • 2COrm
i
_z60 3o
Figure 3
,,, Range distributions for
iron-56 nuclei with energies4o 20
a: 473 and 529 MeV/amuhA
m stopping in the fiber bundle.
° yz 2C IO
, \_.J
2 3 20 21 22
BUNDLEPENETRATIONDEPTH(mrn)
4. Hodoscope Results. To study the positional resolution that we could expect to
obtain with a hodoscope made of scintillating fibers we have calculated, for a single
track, the weighted mean position in fiber layers perpendicular to the track direction.
These layers are adequately separated so that they are optically decoupled from one
another. These points were then fit by a straight line as shown in Figure 4. The rms
deviation of 'these "individual layer means" from the straight line should then give an
indication of the trajectory measurement capability of this technique. Our
measurements show a deviation from the best fit line of about 35 microns rms over the
entire track length and 10 microns over a limited track segment. It seems clear that
the larger deviation over the entire track length (about 200 fiber layers) is the result of
systematic nonuniformities in the fiber bundle and that something approaching l0
microns is the true measuring precision which was obtained. This measurement
precision is actually better than could be obtained if the fibers were optically decoupled
from each other. (If light were detected only from the fibers through which the primary
nucleus penetrates, then 100 micron fibers would give a measurement precision of 30
microns rms.) This is the result of the additional information contained in the fibers
adjacent to the fibers actually traversed by the particle, thus improving our ability to
estimate the "center of gravity" of the track.
2715 OG 9.1-7
I ! I I !
"E I OQ
E3z . "" " "
_o. Figure 4.l--
Weighted mean position in
2 ... the fiber bundle plotted vs.z
,_ the penetration depth intot_
:s / _ the fiber bundle. Each fiber
o layer is I00 microns thick.
hi
0 I I I I , I
O 50 IOO 150 200
FIBERLAYERNUMBER
4. Conclusions. We have developed a new type of electronic detector which is
capable of me,_suring range and trajectory to the precision required for resolving
individual isotopes.
5. Acknowledgements. This work was supported in part by NASA grants NGR-26-
008-001, NAGW-122, and NASA contract NAS5-27996 and in part by the McDonnell
Center for the Space Sciences.
6. References
Binns, W.R., Israel, M.H., Klarmann, J. 1983a, Nucl. Inst. Meth., 216, 475.
Binns, W.R., Israel, M.H., Klarmann, J. 1983b, 18th ICRG, Vol.8, p.89.
Henke, R. P., and Benton, E. V., 1966, Nov., USNRDL-TR-1102.
Webber, W.R., and Kish, J.C., 1983, 18th ICRC, Vol.8, p.40.


Bevalac calibration of the SOFIE range and hodoscope detectors

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