The response of Pilot 425 to heavy ions with energies less than 600 MeV/amu beta approximately 0.8 is examined both theoretically and experimentally. Measurements are presented from an experiment which employed a Ne-20 beam at many energies below 575 MeV/amu. The signal is assumed to come from three sources: (1) Cerenkov light from the heavy ion, (2) Cerenkov light from secondary electrons, and (3) scintillation of the radiator. It is found that the effective index of refraction is 1.518 and that scintillation is present at a level of approximately 2.7 percent of the Cerenkov signal for beta = 1 for Ne-20. The first of these values differs from values previously quoted in the literature

Ahlen, S. P.

Cartwright, B. G.

Tarle, G.

English

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ft
Near Threshold Response of a Wave Shifted
Cerenkov Radiator to Heavy Ions
Steven P. Ahlen, Brian G. Cartwright and Gregory Tarle
Department of Physics
University of California
Berkeley, California 94720
Abstract
The response of Pilot 425 to heavy ions with energies less than
600 MeV/amu (8 z 0.8) is examined both theoretically and experimentally.
Measurements are presented from an experiment which employed a Ne20
beam at many energies below 575 MeV/amu. The signal is assumed to
,
come from three sources: 1) Cerenkov light from the heavy ion;
2) Cerenkov light from secondary electrons; and 3) scintillationi of
the radiator. it is found that the effective index of refraction is
1.518 and that scintillation is present at a level of -2.7% of the
Cerenkov signal for 8 - 1 for Ne 20 . The first of these values differs
from values previously-quoted in the literature.
	
(NASA -CP- 146797) NEAR THRESHOLD RESPONSE OF
	 N76-22002
A WAVE SHIFTED CERENKCV RADIATOR TC HEAVY
IONS (California Univ.) 16 p HC $3.50
	CSCL 20H
	 Unclas
G3/72 25139
!APR 10176	 {
R CEIVED
NASA 0 FACILITY
_N INPUT BRANCI•I
1k h-w -
Introduction
At medium energies Cerenkov counters can provide accurate velocity
measurements for particles with known charge. At relativistic energies
they provide accurate charge measurements with important advantages
over organic scintillators: they do not saturate with Increasing
charge, and they do not suffer from large Landau fluctuations. Since
Cerenkov emission is feeble, good resolution requires that as much
Cerenkov light as possible be collected. One way in which this can
be done is to dope the radiator with a wave-shifting material which
converts high frequency Cerenkov light into lower frequency light which
can more easily escape the radiator and convert to photoelectrons at
the photocathodes of photomultiplier tubes. A quenching material must
then also be added to the radiator to minimize the scintillation of
the wave-shifting fluor. In addition to providing a greater number of
i
	 visible photons, wave-shifted radiators can make collection of a larger
g	 fraction of these photons possible since the wave-shifted photons are
i
emitted isotropically. Roughly half of the total light is.wave-shifted
and hence isotropized. For particle energies near the Cerenkov thres-
hold, many of these isotropized photons will be totally internally
reflected while the directional primary Cerenkov component will not be
(for normally incident particles, total internal reflection of this
component occurs at 8 z 0.89 for an index of refraction of 1.5). To
capitalize on this advantage, one should use adiabatic light pipes to
conduct the light to the photolnultiplier tubes since internal reflec-
tion is required for efficient light piping. Unfortunately, high spatial
uniformity is difficult to attain with light pipes.
-2-
The most readily available wave-shifted radiator, and the one
which will be discussed it this paper, is Pilot 425, manufactured by
Nuclear Enterprises, Inc. of San Carlos, California. Several authors
have investigated the properties of wave-shifted radiators;
1
' 20 in
each case Pilot 425 was among the samples tested. Sacharidis l Inves-
tigated the response of light-piped radiators to singly-charged beams
and hence was hindered by small quantities of light (less than 1 photo-
electron per particle below threshold) and also by a complicated energy
and angle dependence of the experimental light-collection efficiency.
Atallah and Schmidt 2 set limits on the amount of scintillation of wave-
shifted radiators by taking long exposures with extremely sensitive film
while irradiating sample radiators with alpha-particles. They concluded
that scintillation contributes less than 5% of the total light emitted
by relativistic, singly-charged particles. Cantin et al. 3 used a N14
heavy ion beam at the Lawrence Berkeley Bevaiac to examine the response
of a variety of radiators, each housed in a light integration box. For
Pilot 425 they quote an index of refraction of n - 1.49 and a scintilla-
tion fraction of 3% for relativistic N 14 ions. Gilman and Waddington9
have referred to an index of refraction of n - 1.44, a value also
commun i cated to us. t0
Theoretical Response
For any Cerenkov radiator there will always be three contributions
to the total light output: 1) Cerenkov emission from the primary
particle; 2) Cerenkov emission from secondary electrons; and 3) scin-
tillation.
Only the relative contributions of Cerenkov and scintillation
light distinguish a Cerenkov radiator from a scintillation detector.
-3-
-11 
If
NO
dL l dL2 dL
We let dx ` dx ' :' respectively, denote these contributions to
total light output per unit length. For direct comparison with expert-
ment we use units of photoelectrons per unit length.
We restrict our attention to a 1.27 cm thick•Pilot 425 radiator,
since our results may easily be extended to other radiators. We also
restrict our attention to energies below 600 MeV/amu (S = 0.8), since
we have experimental data only for these energies and since at greater
energies prediction of response becomes more difficult because of the
Increasing complexity of the delta-ray transport problem.4
We first consider the Cerenkov emission from the primary particle.
For a non-wave-shifted radiator
dxl = Zvi c3— f	 f 1 9 {w) q (r+►) { 1 -	 n 2 w )	
{ 1)
n(w)>-
where Ze is projectile charge, 8c, projectile speed, f, the light
collection efficiency, q(w), the quantum efficiency of the phototubes,
and g(w), the fraction of light of frequency w which can escape the
radiator without being absorbed. For Pilot 425 we adopt the following
naive model: for wavelengths longer than A 1 , but shorter than A2
primary Cerenkov photons are absorbed by the wave-shifter and converted
Into photons with a wavelength of 425 nm (the peak of the fluorescence
spectrum of pilot 425. FWHM is 50 nm). For wavelengths longer than
A2 primary Cerenkov photons are unaffected by the wave-shifter. With
wi	 2 we have:
i
dL	 Z2 e2
dx l = h-cam f[ fw2 g {w) g tw) tl - S n i w ) dw t jwl g two) q (wo) {1 - 8 n l w ) dw]
o	 w2
(2)
where o • 2nc/425 nm.
The known optical properties of Pilot 425 determine w 1 , w2 and
g(w). Nuclear Enterprises gives the following information regarding
Pilot 425:
Ninimum detectable Cerenkov wavelength 260 nm
index of refraction at 589.2 nm - 1.490
index of refraction at 425 nm 1.502
The light transmittance of Clinical Perspex (UVT polymethylme-
thacrylate)and of Pilot 425 were measured by E.J. Sacharidis j for l cm
thick samples. Clinical Perspex was opaque at 295 nm and showed a
90% transmittance at 325 rim. The corresponding wavelengths for Pilot 425
are 390 nm and 414 nm. The difference between Pilot 425 and Clinical
Perspex is due to the wave-shifter: for wavelengths shorter than
-400 nm the wave-shifter is absorbing, while for longer than 400 nm wave-
lengths the wave-shifter l5
 is transparent. So we will take A 2=400 nm. For Ai
we will use 260 nm, the shortest detectable Cerenkov wavelength (this
Is consistent with the transmittance cutoff of Clinical Perspex). Since
Pilot 425 is transparent to wavelengths longer than 400 nm we set
g(w0 ) - g(w) - g. For the index of refraction we assume the form
obtained for the elementary classical model of a collection of damped
oscillators:
n (w) - 1 + w
	
.
	 (3)
0
Using the above indices for Pilot 425, we obtain C - 1.931 x 1032/sec2,
100Z - 4.044 x )032/sec2.
Using the quantum efficiency for RCA 4525 tubes (which were used
In the experimental tests) we obtain:
-5-
i
dL
 = fg Zj (7.60 x 10 14lsec) 0	 1	 )dx	 he	 020-515
' nZ2 0 t3 2-1 2	 (4)
no
Here we define n, ,_the system figure of merit and no , the effective
Cerenkov index of refraction.
It is a straight forward problem to evaluate the contribution due
to the Cerenkov emission of secondary electrons. The number of delta
rays produced per unit energy a per unit length by a heavy particle
of charge Ze, velocity 8c is:
dedx meN2s2 C2 
(1-82 
em)
	
(5)
where N is the electron number density and em
 is the kinematic limit
for energy transfer to an electron: em - 2mecYY 2 . We will assume
that each electron of energy a remains in the radiator at least until
Its energy drops below the Cerenkov threshold. This is certainly a
good approximation for 8 :9 0.8 (at this velocity the practical range
of the maximum energy delta ray is roughly I the thickness of a 1.27 cm
radiator). The relatively small number of delta rays which escape the
radiator above threshold will be replenished by delta rays produced
In the material above the - radiator. For velocities in excess of 0.8 c,
delta rays are produced which can easily escape the radiator. Evanson4
treats this problem by introducing the concept of a cutoff energy E 
below which delta rays contribute according to our prescription and
above which they behave as though they have energy E c . For our purposes
this additional complication is unnecessary. Hence, we have that each
secondary electron of energy a contributes an amount of light
-6-
WWI-
kREPRODUCIBILITY OP THE
e0 - 1	 de	 ORIGINAL PALL IS POOR.
j	 (i a 2n 2 de , e being the threshold energy for Cerenkov
e	 e o dx	 o
emission by an electron. For dx we use the tabulated values of Bercer
and Seltzer. 5 These results include bremsstrahlung losses and follow
the higher energy electron for high energy transfer collisions. For
a given medium and index of refraction,
dL
dxZ = 
nZ2 f (S)
f(S) is sketched in Fig. 1 for n o = 1.5 and the value of N corresponding
to polymethyimethacrylate (N = 3.88 x 102313). It is seen that
dL
between Q = 0.575 and 8 = 0.8, dx2 is a linear function of 8. dL
	
It is impossible to calculate the scintillation component	 3dx
a prio ri since scintillation response varies with both dx and Z, i.e.,
scintlllators "saturate." The total "pure" Cerenkov yield is obtained
dl 1	dl2
by integrating dx + dx through the radiator thickness.
dL 1
Consider for a moment dx only. By changing the variable of inte-
gration to a dimensionless momentum  p = i3Y we have:
dL	 p	 pAm c2
fdxl dx = L1 nZ 2 (1 nom) j o (1	 n ^ 1 p ) ( d--E^—)dp
	
(7)
	
o p i	 o TX
where m  is the proton mass, A is the mass number of the nucleus and
where p i (p0) is the initial (final or threshold) momentum. Considering
the quantity pl(dxY) to be constant one obtains:
L1 ' 
nZ2(1 - nom) t(1 - n -i p p )0	 0	 of
where t is the radiator thickness (or the depth of penetration at which
the particle drops below threshold). One can show that if a represents
(6)
(8)
-7-
pAm c2
the peak-to-peak fractional variation of the quantity d— Y then
dxY
L i = 11Z2  - n-^-) t0 - n {1;ai p )
	 (9)
0	 o	 po i
For Ne 20 ions this implies an accuracy of 0.1% for equation (8).
Unfortunately, we know of no a ,:^ivate approximation for
t dL2
L2 s 
^ dx dx, which must be evaluated numerically. in Fig. 2 we
0
plot (L 1 + L2) for Ne20 incident normally on a 1.27 cm thick Pilot 425
radiator for various indices of refraction. Similar curves are pre-
sented for Fe". Note that the extrapolated cutoff energy for iron
is enhanced relative to neon. This is due to the greater slowing rate
of the heavier nuclei.
Experiment
We used the Cerenkov counter described in ref. 7 to perform tests
of the Pilot 425 response. This counter consists of a sandblasted
radiator placed in a light integration box, viewed by 16 RCA 4525
photomultiplier tubes. The dounter is characterized by exceptional
spatial uniformity and light collection efficiency and an absence of
any angular dependence or energy dependence of the light collection
efficiency (in the previous section this was an implicit assumption
since g did not depend on 0 or the angle of incidence of the particle;.
The tests were performed at the Lawrence Berkeley Bevalac where we had
a Ne20
 heavy ion beam with an energy of 594 MeV/amu. The beam was
focused to a diameter of less than 2 cm. Matter in the beam line and
In the top of the light integration box reduced the beam energy to
572.4 MeV/amu at the top of the Pilot 425 radiator. A 1024-channel
-8-
analyzer was used to accumulate pulse height spectra at this and at
lower energies (obtained by degrading the beam with lead absorbers).
Fig. 3 displays the response as a function of energy for normal incidence.
The energies for the data points were obtained by using the range-energy
program of Henke and Benton. 8 The error bars for the iow energy data
are indicative of the magnitude of a possible systematic error due to
uncertainties in the beam energy (or equivalently due to uncertainties
In the matter in the beam line) of ±1.5 MeV/amu at the exit window into
the experimental area. For energies above -100 MeV/amu this systematic
error is negligible. The ordinate is expressed in units of photoelec-
trons. The scale was determined from the width of the maximum energy
data points (multidynode counting statistics were taken into considera-
tion). Note that below threshold, the response is remarkably constant.
This cannot be explained in terms of scintillation alone. However,
the combination of a term varying as _1/8 2 with a term varying as -8
(from delta-ray Cerenkov radiation) produces this flat curve. The
effective Cerenkov index of refraction n o , and the system figure of
merit n were obtained by an iterative procedure. We first estimated
n to be 37.0/cm from results of spectra obtained with atmospheric muons.
We also estimated no to be 1.525 from preliminary analysis of the data.
We calculated the pure Cerenkov response (primary particle plus secon-
dary electrons) and subtracted this from the observed curve. This
gives an estimated scintillation response which is quite accurate in
the region below threshold since the delta-ray Cerenkov contribution
Is insensitive to small changes Inn and n o. We then compared the
scintillation response in this accurate region with the response from
If,
-9-
a Pilot Y scintillator and found the saturation properties to be quite
similar. This enabled us to extend the estimated scintillation response
out to an energy of 600 McVlamu. Subtracting this response from the
observed response produced an experimentally determined "pure" Cerenkov
curve which was then fit very nicely with an Index of n o - 1.515. By
repeating this procedure one more time we obtained the following values
for no and n:
no - 1.518 t .005
n - 35-3/cm
In Fig. 3 we plot the scintillation and Cerenkov response
separately.
Our value of no does not agree with either of the values 1.449110
and 1.49 3 which are often quoted. if one extrapolates the above thres-
hold response for the proton results of ref. 1 one obtains an effective
Index of refraction of 1.40. This latter result can be explained by
Inefficient light collection at near threshold velocities where much
of the light is In the forward direction and hence has little chance
of being totally internally reflected and subsequently collected. Uni-
form light collection is essential in any attempt to ascertain the
energy-dependence of the response of Cerenkov radiators. Problems such
as these perhaps account for the discrepancies of the earlier results
from both our measured and theoretical indices (1.518 and 1.515 respec-
tively) which agree very well.
By a comparison of the scintillation response to the Cerenkov
response for B % 1 (obtained by extrapolating from low velocities) we
find a scintillation-to-Cerenkov emission ratio of 2.7%, consistent
with previous results. 1,2,3
I
-10-
Acknowledgments
We wish to express our appreciation to P. Buford Price for his
.ipport of this work, and for the excellent environment for scientific
research which exists in his group. We thank the staff of the Lawrence
Berkeley Laboratory Bevalac for their kind assistance in arranging for
our exposures. This work was supported by NASA Grant NGR 05-003-376.
-11-
.	
References I%
1) E.J. Sacharidts, Nucl. Instr. and Meth. 101 (1972) 321.
2) K. Atallah and W.K.H. Schmidt, Nucl. Inttr. and Meth. 120 (1974)
539.
3) M. Cantin, P. Goret, J. Jorrand, R. Jouan, E. Juliusson, L. Koch,
Y. Maubras, P. Mestreau, N. Petrou, Y. Rio, A. Soutoul, Proc. 144th
Inter. Cosmic Ray Conf., Munich 	 (1975) 3209.
4) P. Evanson, Proc. 14th Inter. Cosmic Ray Conf., Munich 4 (1975)
3171.
5) M.J. Berger and S.M. Seltzer, Tables of Energy losses and Ranges
of Electrons and Positrons, NASA SP-3012 (1964).
6) M. Cantin, P. Goret, J. Jorrand, E. Julliusson, L. Koch, Y. Maubras,
P. Mestreau, N. Petrou, A. Soutoul, Proc. 14th inter. Cosmic Ray
Conf., Munich .1
 
(1975) 3205.
7) S.P. Ahlen, B.G. Cartwright, G. Tarle, submitted to Nucl. Inttr.
and Meth. (1976).
8) R.P. Henke and E.V. Benton, "A Computer Code for the Computation
of Heavy-Ion Range-Energy Relationships in any Stopping Material,"
U.S. Naval Red. Defense Lab. TR-67-122 (1967).
t
9) l.M. Gilman, C.J. Waddington, Proc. 14th Inter. Cosmic Ray Conf.,
Munich, ,j (1975) 3166.
10) A. linney (1974), private communication.
-12-
Figure Captions
Figure 1. light output due to Cerenkov emission from delta rays. The
number of photoelectrons collected per unit distance
•	 traveled by the primary particle of charge Ze is equal to
nZ2M) where n is the light collection system figure of
merit.
Figure 2. Pure Cerenkov emission from both the primary particle and
secondary electrons for ions traversing a 1.27 cm thick
Pilot 425 radiator. Several indices of refraction :ave
been used for the calculations to illustrate the nature of
the dependence of the light output on this parameter. The
ordinate is in units of photoelectrons and n is expressed
In terms of photoelectrons/cm. The top curve (a) corresponds
to Z - 14, no R 1.53, the middle curve (b) to Z - 10;
no
 
a
 1.515, and the bottom curve (c) tc; Z - 26, no ; 1.515.
Figure 3. Observed response of the Cerenkov radiator as a function of
Incident energy. The theoretical pure Cerenkov response and
the scintillation component are plotted separately.
3
OD
D
f^
Q
t00
oCo.
0
M
O
N
O
O
N
	 0	 OLO
O	 p	 C5	 Q0
a
b
C
Q
0.4
0.3
C*j
-i IN
Ql
0L
0 IUO 200 300 400 500 600
E (MeV/amu)
Q2
OBSERVED
RESPONSE
i
cn
1200
ZO
oc
UWJ0 800F-0
a
THEORETICAL
CERENKOV
t
1600
400
D	 SCINTILLATION
200	 400	 600
INCIDENT ENERGY (MEV/AMU)


Near threshold response of a wave shifted Cerenkov radiator to heavy ions

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