PFC/JA-92-5
MIT Fusion 7-Ray Diagnostic Development
K. W. Wenzel, R. D. Petrasso, D. H. Lo, C. K. Li,
J. W. Coleman, J. R. Lierzer, E. Hsieht and T. Bernatt
Submitted for publication in: Review of Scientific Inatruments
Plasma Fusion Center
Massachusetts Institute of Technology
Cambridge, MA 02139
f Lawrence Livermore National Laboratory
Livermore, CA 94550
This work was supported in part by U.S. DOE Grant No. DE-FG02-91ER54109
and LLNL Subcontract No. B116798.
MIT Fusion y-Ray Diagnostic Development
K. W. Wenzelt, R. D. Petrasso, D. H. Lot, C. K. Li,
J. W. Coleman, and J. R. Lierzer,
MIT Plasma Fusion Center, Cambridge MA 02139
E. Hsieh and T. Bernat
Lawrence Livermore National Laboratory, Livermore CA 94550
March 1992
Abstract
In conjunction with -y-ray diagnostic development for the NOVA Upgrade
and Alcator C-MOD experiments, we have assembled a comprehensive set of
absolutely-calibrated -y-ray sources and several y-ray detectors. These tools
will be used in characterizing and calibrating new y-ray diagnostic systems
as they are assembled. The 7-ray sources comprise both radioisotopes and
nuclear reactions. The latter are generated in the MIT Cockcroft-Walton
fusion-product generator. The detectors include several "standard" NaI(Tl)
scintillators and a high-resolution germanium detector. This paper briefly
discusses the diagnostics planned for NOVA Upgrade and Alcator C-MOD.
In addition it describes the 7-ray sources and detector characterizations we
have performed in the laboratory.
1
I. Introduction
-ray diagnostics have certain advantages over other fusion-product diagnostics, because -y rays escape the fusion plasma and its vacuum vessel
without suffering significant scattering. Charged particles, by contrast, are
largely confined in the plasma, and neutrons are scattered by the surrounding structure. Thus the -y-ray branches of fusion reactions provide a unique
opportunity to measure the fusion yield and the fusion burn history.
We are developing 7-ray diagnostics for the NOVA Upgrade ICF experiment and the Alcator C-MOD tokamak experiment. The primary objectives
are measurements of the 7-ray yields from the T(D,y)"He and 3 He(D,-y) Li
reactions at 16.7 MeV and 16.4 MeV, respectively. Since the y-ray branching
ratios are known (though small; see Table 1),4' a measurement of the y-ray
yield is also a measure of the total fusion-reaction yield. The Y emissions
from the "mirror" D-T and D- 3 He reactions are nearly identical, because the
nuclear force is charge symmetric, and the product "He and "Li nuclei have
similar energy levels with very broad first excited states.4 Section II below
describes the concepts for the NOVA Upgrade and Alcator C-MOD 7-ray
diagnostics.
In order to successfully develop these diagnostics, we have established a
comprehensive set of y-ray sources and detectors. The sources, both radioisotopes and nuclear reactions, provide y rays of known intensity over an energy
range of 0.1 MeV to 17 MeV and are described in Section III. The detectors
2
include scintillation detectors and a high-resolution, High-Purity Germanium
(HPGe) crystal, and they are described in Section IV. Together, these tools
will be used to fully characterize and calibrate new -y-ray detection systems.
II. -y-Ray Diagnostics for NOVA Upgrade and Alcator C-MOD
Measurement of the high-energy D-T y rays in the NOVA Upgrade experiment is the only practical way to determine the fusion burn history. The other
fusion products cannot be accurately used for this purpose; the neutrons are
significantly Doppler broadened, and the charged particles are stopped in
the high-density target plasma. Since the burn width in the NOVA Upgrade
experiment will be short (- 500 ps), the D-T -y rays can easily be separated
from the neutrons because of their disparate arrival times at the detector.
Table 2 illustrates this by listing several pertinent times for a detector at
distance d from the target: the 14 MeV neutron arrival time at the detector;
the spread (FWHM) in neutron arrival times due to energy Doppler broadening (AE ~ 177 T1(keV) calculated for a 10 keV ion temperature using
the formula of Brysk7 ); and the -- ray arrival time at the detector. The y
rays have essentially zero Doppler broadening.
For this measurement we have proposed either a -y-to-pair-to-Cerenkov
spectrometer or a -y-to-pair spectrometer with high time resolution (~
20
ps). The former diagnostic is similar in spirit to that attempted by Lerche,
et a0..
An unwanted "background" of -y rays may be generated by the 14
MeV neutrons undergoing inelastic (n,n'y) reactions in the hohlraum, target,
3
and stem. (In high Z materials, the (n,n'y) cross sections are typically about
2 orders of magnitude higher than (n,-y) cross sections.) It is essential to
understand this contribution to the y-ray signal, which might amplify the
burn signal but would make an absolute D-T yield calibration impossible.
In the case of the Alcator C-MOD tokamak, a -y-ray diagnostic for measuring the high-energy -y rays from D-*He reactions does not require the fast
time response needed for the NOVA Upgrade diagnostic, because the pulse
is much longer (- 1 s). Neither does it require the superior energy resolution of germanium detectors, because the y-ray spectrum is naturally quite
broad. (The 16.4-MeV -y is ~1.5 MeV wide.') Therefore we plan to deploy
a "standard", large (6" x 6") NaI(Tl) scintillation spectrometer on Alcator C-MOD to measure the absolute D- 3 He reaction yield. This information
will complement active (SBD 9 ) and passive (activationo and track detectors)
charged-particle diagnostics.
In the Alcator C-MOD experiment, capture of 2.5 MeV D-D neutrons in
the structure will generate a considerable background of -y rays. Shielding
against these lower-energy -y rays and against the neutrons will be crucial.
We are designing a suitable shield, and once it is assembled we will test its
performance using our Cockcroft-Walton fusion-product generator 1 , which
is a well-calibrated source of 2.5 MeV D-D neutrons.
III. -y-Ray Sources
We use many different -f-emitting radioactive isotopes in our laboratory
4
as calibration standards. The most useful are
22
Na, 137CS,
54
Mn, and 6*Co,
because they emit only single -y lines or well-separated lines that are easily resolved by standard Nal(Tl) scintillation detectors. Other sources (e.g.
1'3Ba
and
220Ra)
emit many 7-ray lines that can be resolved only with the
HPGe detector.
The sources listed above are limited to 7-rays below 3 MeV, far below
the ~16.5 MeV lines from D-He and D-T reactions. Therefore we have also
used an
24
Am/Be neutron source as an absolute source of 4.44-MeV y rays.
These are generated by the reaction 9 Be(a, n)1 2 0*, where the 12C* nucleus
is in the first excited state, which decays by 4.44 MeV 7-ray emission. We
calculated the y-ray intensity from this source using the
mCi), the number of a-"Be reactions per
24
'Am
2 4 1Am
activity (10
a decay, and the fraction of
reactions that produce the 120* first excited state. The second quantity is
n/a c 7 x 10-5,12 and the third quantity (R,) was calculated from the yields
of each
120
final state (0, 1, and 2). These yields were calculated using the
cross sections of Ref. 12, and taking into account a-particle slowing down
in beryllium using the stopping-power formulas of Ziegler13 . The final result
was R,=0.57, in good agreement with the measured value of 0.591.14 Using
R,=0.57 gives an absolute intensity of 4 x 10"
y rays per a decay.
Because the 120* nucleus is extremely short lived (61 fs)1 , the 4.44-MeV
7 ray is quite broad. We therefore fit the measured spectrum to a function
based on the physics of the 7-ray interaction in order to accurately determine
5
the total peak counts for an absolute efficiency measurement (Fig. 1).
In order to test our -y-ray detectors at even higher energies, we use several
nuclear reactions in our Cockcroft-Walton fusion-product generator 1 (see
Table 1). Of particular interest to us is the 1 'B(p,-y)
C reaction, which yields
-y rays at 4.44, 11.5, and 16 MeV. The thick-target 7-ray branching ratios
for "B(p,y)
12 C
have been measured15 , so by measuring the corresponding a
particles from the 'B(p,a)aBe reaction we establish the "B(p,y)
12 C
reaction
as a high-energy 7-ray source of well-known absolute intensity.
Fig. 2 shows the p-"B 7-ray spectrum measured with a 3" x 3" NaI(Tl)
detector while a pressed boron target was bombarded with a 170-kV, 25-IA
proton beam (inset shows the a-particle spectrum). This beam energy is
sufficient to access the p-l'B reaction resonance at 163 keV. Similar data
were used to calculate the absolute peak efficiency for our HPGe detector at
4.44 and 11.5 MeV (Fig. 3).
IV. 7-Ray Detector Characterizations
We use four different 7-ray detectors in the laboratory: two "standard"
3" x 3" NaI(Tl) detectors, a 5" x 3" NaI(Tl) detector, and a 200-cms (50%
relative efficiency) HPGe detector. We are using the sources described in Section III to fully characterize and calibrate these detectors. For example, Fig.
3 illustrates the absolute peak efficiency measured for the HPGe detector.
6
Acknowledgements
Dr. Ramon Leeper and Dr. Carlos Ruiz of Sandia National Laboratory
provided detectors used in this work. We acknowledge Dr. Ed Cecil of the
Colorado School of Mines for helpful discussions, and we thank Dr. Fredrick
Seguin and Cristina Borris for their careful reading of the manuscript. This
work was supported by LLNL Subcontract B116798, U.S. DOE Grant No.
DE-FG02-91ER54109, by U.S. DOE Fusion Energy Postdoctoral Fellowshipt,
and by U. S. DOE Magnetic Fusion Energy Technology Fellowshipt.
7
References
1. S. S. Medley, F. E. Cecil, D. Cole, M. A. Conway, et al., Rev. Sci.
Instrum. 56 975 (1985).
2. G. Sadler, 0. N. Jarvis, P. v. Belle, et al., in Controlled Fusion and
Plasma Heating (Proc. 14th Eur. Conf., Madrid, 1987), 11D, Part III,
p. 1232 (1987).
3. R. D. Petrasso, C. L. Fiore, and C. K. Li, Rev. Sci. Instrum. 59 1721
(1988).
4. J. E. Kameraad, J. Hall, K. Sale, et al., submitted to Phys. Rev. C
(1991).
5. F. E. Cecil, D. M. Cole, R. Philbin, Phys. Rev. C 32 690 (1985).
6. F. E. Cecil and F. J. Wilkinson III, Phys. Rev. Lett. 53 767 (1984).
7. H. Brysk, Plasma Phys. 15 611 (1973).
8. R. A. Lerche, M. D. Cable, D. W. Phillon, Rev. Sci. Instrum. 61 3187
(1990).
9. D. H. Lo, R. D. Petrasso, K. W. Wenzel, et al., submitted to Rev. Sci.
Instrum. (1992).
10. J. R. Lierzer, R. D. Petrasso, K. W. Wenzel, et al., submitted to Rev.
Sci. Instrum, (1992).
8
11. K. W. Wenzel, D. H. Lo, R. D. Petrasso, et al. submitted to Rev. Sci.
Instrum., (1992).
12. K. W. Geiger and L. van der Zwan, Nucl. Instrum. and Meth. 131
315 (1975).
13. J. F. Ziegler, Helium: Stopping Power and Ranges in all Elemental
Matter, Pergamon Press, (1977)
14. S. Croft, Nucl. Instrum. and Meth. A281 103 (1989).
15. F. E. Cecil, D. Ferg, H. Liu, et al., to be published in Nucl. Phys. A
(1992).
16. R. Gunnink, Nucl. Instrum. and Meth. A299 372 (1990).
17. F. E. Cecil and S. S. Medley, DOE-APP alpha particle workshop, Germantown MD, Dec. (1989), unpublished.
9
Table 1: Ground-state fusion reactions that generate high-energy -y rays.
Refs.
-y-Ray
E.,
-t-Ray
Reactants Particle Q
I Branch
(MeV)
Products
p+D
p+T
-
-
-
-
p+5Li
p+7Li
p+ 9 Be
p+"B
YHe+a
a+a
a+aLi
c+sBe
Y+"Be
4.02
7+'Be
17.35
y+ 10 B
2.12
+1 2 C
8.59 '
3a
8.68
p+T
4.03
n+He
3.27
n+a
p+a
a+ a
a+5 He
17.59
18.35
22.37
14.22
D+D
D+T
D+ 3 He
D+ 6 Li
D+ 7 Li
f+a
(MeV)j B.R.
5.5
oo
o
19.8
1
1
5.61
17.25
6.59
15.96
2.4 x 10-1
3 x 10-3
2.9 x 106.5 x 10-r
15
15
15
15
Y+ a
23.85
1.0
1
-+He
y+ 5 Li
Y+'Be
-+ 9 Be
16.69
16.38
22.28
16.70
5.4x 104.5 x 10"
< 10-g
< 7 x 10-B
10
x
10-7
6,1
5
17
17
Table 2: For a 10-keV ICF plasma, this table compares the arrival time (t")
of the 14-MeV neutrons, the spread (FWHM) in their arrival times (At), and
the arrival time (t.) of -y rays at a detector located a distance d from the
target. A burn width -500 ps is expected in the NOVA Upgrade.
d
t"
At |t
(cm)
I(ps)
10
100
1000
1,926.78
19,267.8
192,678.
I(ps)
38.24
382.4
3824.
11
(pS)
333.567
3,335.67
33,356.7
'U-
104
12 0
-
4
U 102-
208
TI 24 1Amn/Be
100 -:100
80
0
1000
3000
4000
Energy (keV)
o
-
2000
5000
-
-
-
-o
6
40
0
* . Wool'
.
40.*-....*..'..*-*.
-
20
20
0
'
. .' I''
'
4000 4100 4200 4300 4400 4500 4600 4700
Energy (keV)
Fig. 1. The y-ray spectrum from a 10 mCi 24 1Am/Be source was obtained
with our 200-cm3 HPGe detector. The portion of the spectrum around the
4.44 MeV peak (inset shows the entire spectrum) was fit to the function
a, e-(Z-2/a+ a4e-(B-aj+ a7 E +as. In this fit, the low-energy gaussian
represents the Compton edge, and the high-energy gaussian represents the
full-energy peak. Total counts derived from this fit were used to determine
the absolute peak efficiency of the HPGe detector at 4.44 MeV (Fig. 3).
12
"3
15
'
300
11 B(p,a)2a
250
C
200
-01U.
ISO
104:_
1
100
I B(p,a)880'
oso50
JC
0
10 3
,
2
3
4
6
7.
Energy (MOV)
L
M o2
0-10
0
101
100
0
2
4
6
10
8
12
14
16
18
20
Energy (MeV)
Fig. 2. A p-l'B 7-ray spectrum was taken with a 3" x 3" NaI(TI) detector
over two hours while we bombarded a boron target with a 170-kV, 25-pA
proton beam (inset shows the corresponding a-particle spectrum). Similar
data were used to measure the absolute peak efficiency of the HPGe detector
at 4.44 and 11.5 MeV (Fig. 3).
13
10-1
d=10 cm a 1 =-5.8268733:
a 2 =-0.7102472
a3=
C
0.0490545
a 4 =-0.0820222-
10-2
a5=-0.0040060
a -~0.0003374
a 10-3
CL
0
10-5 1-eeti
0.1
::
y:.sna
10
1
100
Energy (MeV)
Fig. 3. Absolute peak efficiency (peak counts per source photon) was measured for our HP Ge detector at a source-to-detector distance of 10 cm for y
energies from 0.19 to 11.5 MeV (data points; the 4.44 MeV point from p-"1 B
reactions is offset to 5.44 MeV for clarity of display). We fit these data to
a function, ln(r1) = E£?.a[ln(E)]("1 ), a common form for solid state detectors for energy >200 keV.1 The uncertainties in these data were derived by
adding in quadrature the uncertainties in absolute source intensity, geometry,
and counting statistics.
14