Gamma-ray spectrometry provides diagnostics of fast ion behavior in plasmas of large tokamaks. Information acquiring with the gamma-ray diagnostics gives possibility to identify and distinguish simultaneously presence of fast alpha-particles and other ions He), to obtain its relative densities and also to perform tomographic radial profile reconstruction of the gammaemission sources

A E Shevelev

D B Gin

I N Chugunov

V G Kiptily

CiteSeerX

GAMMA-RAY SPECTROMETRY OF HOT PLASMAS 
I.N.Chugunov1, V.G.Kiptily2, A.E.Shevelev1, D.B.Gin1 
1 Ioffe Physical-Technical Institute of the Russian Academy of Sciences 26, Polytekhnicheskaya, St Petersburg 
194021, Russian Federation, Chugunov.cycla@mail.ioffe.ru 
2Euroatom/CCFE Fusion Association, Culham Science Centre, Abingdon OX14 3DB, UK 
Gamma-ray spectrometry provides diagnostics of fast 
ion behavior in plasmas of large tokamaks. Information 
acquiring with the gamma-ray diagnostics gives 
possibility to identify and distinguish simultaneously 
presence of fast alpha-particles and other ions (H, D, T, 
3He), to obtain its relative densities and also to perform 
tomographic radial profile reconstruction of the gamma-
emission sources. 
I. INTRODUCTION 
Diagnosis of alpha-particles and other fast ions, 
which are born in nuclear reactions during ICRF and NBI 
heating is needed for heating scenario optimization in 
large fusion machines like the JET tokamak. For ITER, 
which is under construction this moment this problem is 
of a crucial importance. Gamma-ray spectrometry 
techniques allow studying the behavior of energetic ions, 
including fusion alpha-particles in fusion plasmas 
providing important information on spatial and energy 
distribution of fast particles. More than 20 years gamma-
ray diagnostics is developed in the Ioffe Institute (Ref. 1).  
Gamma-ray diagnostic is included into the 
implementation list of ITER diagnostics. 
Main principles of gamma-ray diagnostics, as well as 
some experimental results and  conceptual design of the  
ITER  gamma-ray spectrometer system are briefly  
represented in this paper. 
II. BASICS OF THE GAMMA-RAY DIAGNOSTIC 
TECHNIQUE 
II.A. Physical Basis 
Gamma-ray spectrometry in hot plasma experiment is 
based on registration  and analysis of radiation from 
nuclei excited in reactions that can take place during 
plasma discharge (Ref. 2). There are three types of 
nuclear reactions that may be noted for diagnostics: 
reactions between fuel nuclei (H, D, T, 3He), interaction 
between fast charged particles (p, d, t, α, 3He) and plasma 
impurities (9Be, 12C) and reactions with specially doped 
impurities (10B, 11B, 7Li). The primary application of the 
diagnostics is the measurement of time-dependent spatial 
gamma-ray and neutron emissivities to derive (i) fusion 
reactivity and alpha-particle birth profile by means of 
reactions D(t,γ)5He and D(t,n)4He; (ii) profile of fast 
confined alpha-particles  - 9Be(α,nγ)12C and 10B(α,pγ)13C; 
(iii) D/T fuel ratio - D(d,γ)4He and D(t,γ)5He; (iv) degree 
of the fast ion confinement under ICRF or/and NBI 
heating; (v) profile of runaway electrons from 
bremsstrahlung hard x-rays. 
  Gamma-ray emissivity of fusion plasma is defined 
by a value of the reaction rate  
    v
rnrn
rR σ
δ12
21
1
)()(
)(
+
= ,                           (1) 
where <σv> 
  
is rate parameter averaged over the 
Maxwellian distribution of  fusion particles; n1, n2
  
are the 
densities of reacted ions and δ12 is Kronecker’s symbol. In 
the case of fast ion reactions the rate is expressed as 
∫⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
= dErEFEErn
m
rR it
i
i ),()()(
2
)( 2
12
1
σ ,       (2) 
where  Fi(E,r) is the fast  ion distribution function;  nt(r) is 
a density of the impurity target. 
A spectrometer, placed in a collimator views a 
plasma volume along the line of sight. The gamma-ray 
count rate can be written as 
   ∫ Ω= LOSdVREEkP )()( γγ ε ,                   (3) 
where   k(Eγ) is a gamma-ray attenuation factor in the 
LOS,    ε( Eγ) -  the detector efficiency; Ω - the detector 
solid angle, VLOS the plasma volume viewed by the 
spectrometer. 
It should be noted, that the reaction 9Be(α,nγ)12C is the 
main for the fusion-born alpha-particle measurements, the 
significance of which was investigated in detail (Ref. 1). It 
is a resonant reaction which also has thresholds. The 
presence of the 4.44 MeV peak in the gamma-ray spectra 
is evidence of the existence of alphas with energies that 
exceed 2 MeV. This reaction was successfully used at 
JET for the fast 4He ions and fusion alpha-particle studies 
(Ref. 3). 
II.B. Gamma-Ray Detectors 
The gamma ray detector  operated in the hot plasma 
experiment must have reasonable energy resolution and be 
capable of maintaining the resolution while operating at 
high-count rates. In addition, the detector should have a 
176 TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY        VOL. 59        JAN. 2011
high enough detection efficiency to allow a statistically 
significant  number  of  counts  be  measured  in  a 
relatively short time so that the time evolution of fusion 
gamma ray production may be reconstructed over the 
course of a single plasma pulse. Finally, the detector 
should be able to withstand a moderate flux of neutrons 
that is associated with a broad class of fusion plasmas. At 
the moment it is possible to choose between two classes 
of gamma ray detectors: scintillators or semiconductor 
detectors.  
Semiconductor detectors (primarily germanium) offer 
superior energy resolution compared with scintillation 
detectors, but they have lower efficiency at high energy 
gamma-ray registration. Also,  the cost of the larger volume 
semiconductor detectors is considerably higher than of 
scintillation detectors. Finally, semiconductor detectors are 
significantly more sensitive to performance degrading 
neutron damage than scintillation detectors. However, the 
JET experiments with fast 4He-ions in the 4He-plasma 
have shown (Ref. 4) that there is a possibility to use the 
specific mechanism of 9Be(α, nγ)12C reaction for α-particle 
study, measuring the Doppler broadened line shape of the 
4.44 MeV gamma-line in the spectrum with high energy 
resolution Ge detectors. 
A number of scintillation materials, such as NaI(Tl), 
Bismuth germanate (BGO), CsI(Tl) are presently utilized 
in the measurement of gamma rays in general and fusion 
gamma rays in particular. These scintillators provide high 
efficiency of registration together with suitable energy 
resolution. However, comparatively long scintillation decay 
times of these materials (230 ns for  NaI(Tl), 1μs for 
CsI(Tl)) makes them unusable for measurements in MHz 
count rate range. New heavy scintillators with densities 
from 5.3 g/cm3 to 8.3 g/cm3: LSO, LYSO, LuAP and 
LaBr3(Ce) are now available for gamma-ray spectroscopy. 
These are rather fast crystals with a decay constant around 
40 ns. The LaBr3(Ce) - ‘BriLanCe’ (Saint-Gobain Crystals) is the most 
attractive one for fusion research. This scintillator has short 
decay times of 16 ns and high photons yields of 63 keV-1, 
(NaI(Tl) scintillator - 38 keV-1). Its properties open a 
possibility to extend the counting rate limit, and at the 
same time to improve the energy resolution for γ-ray 
spectrometry in the range 2-30 MeV. 
II.C. Digital Signal Processing and Data Acquisition 
System 
Since the intensities and the shapes gamma-ray 
spectra can change essentially and rapidly in the course of 
plasma discharge, it is important that the spectrometer 
characteristics, such as energy resolution and the 
efficiency of the spectrometer, as well as the gain of the 
measuring channel,  must be held constant over a wide 
range of counting rates. The time resolution (i.e., the 
minimum time it takes for a spectrum with an adequate 
statistical significance to be acquired) is governed by the 
detection efficiency and the relative counting loss. The 
time resolution can be improved by using fast-response 
high-efficiency spectrometers and high-speed electronic 
equipment.  
A traditional approach includes spectrometric analog-
to-digital converters (ADCs) in gamma-ray spectrometric 
systems, since they are characterized by high integral and 
differential linearity. However, these ADCs have 
significant drawbacks: the instrumental dead time and the 
fundamental impossibility of separating superimposed 
pulses. The gamma-ray detection efficiency of such 
systems starts substantially declining when the 
spectrometer counting rate increases to the level such that 
the percentage of superimposed pulses becomes essential.  
To improve the counting rate characteristics, a new data 
acquisition system (DAQ), exploiting new signal 
processing technique, has been developed in the Ioffe 
Institute (Ref. 5). It is based on a 2-channels 14 bit PCI 
transient recorder for digitizing incoming signal at a 
sampling rate of 25 MHz with an maximum average pulse 
rate of 1MHz. The module provides 2 Gbytes of on-board 
memory, which covers the acquisition time during the 
JET discharge. The recorded data is processed by a special 
code after the discharge. The main advantages of the 
developed system are PHA at high counting rates and gain 
stabilization that is crucial at the fast count-rate variations. 
It is important to note that the DAQ system allows 
avoidance of the pile-up effects, which distort the gamma-
ray spectra, and may cause a misinterpretation of the data. 
II.D. Radiation shielding 
As it was discussed above, the gamma-ray 
spectrometry diagnostics is based on detection and 
subsequent analysis of gamma-ray deexcitation spectra of 
discrete nuclear states excited by nuclear reactions in the 
tokamak plasma. High efficiency detectors measure 
gamma-rays passing through the collimator that selects 
the plasma region for investigation and reduces the 
undesirable radiation flux. Thermonuclear neutrons are 
the main source of background in gamma-ray diagnostics; 
the average energies of neutrons in deuterium (DD) and 
deuterium–tritium (DT) plasmas are 2.5 and 14.0 MeV, 
respectively. Therefore, the radiation shielding of the 
spectrometer used in the gamma-ray diagnostic system 
must be designed so that the signal-to- background ratio is 
at a maximum and the total radiation flux incident on the 
detector does not exceed the admissible value in order to 
avoid uncontrolled distortion of information. 
For the reduction of the intensive neutron fluxes that 
lead to additional detectors load it has been developed a 
6LiH neutron attenuator concept (Ref. 6).  The attenuator 
is a sealed cylindrical capsule (diameter 30 mm, length 
300 mm) with 1-mm-thick walls, which is used for 
plugging the collimator.  The capsule contains lithium 
Chugunov et al.        GAMMA-RAY SPECTROMETRY FOR HOT PLASMAS
TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY        VOL. 59        JAN. 2011 177
hydride (6LiH) enriched to 92% in 6Li isotope and pressed 
into five pellets. The attenuator prototype has been 
successfully tested using a neutron generator and in JET 
DD plasma experiments. The neutron flux attenuation by 
this filter was measured with a neutron generator in the 
modes  corresponding to DD and DT neutrons. Neutrons 
were  detected using the pulse shape discrimination 
technique with thresholds of >2 and >10 MeV, 
respectively. The measured attenuation factors for 2.8- 
and 14.8-MeV neutrons were 900 and 30, respectively. 
Gamma-ray measurements carried out on the JET 
tokamak have demonstrated the efficiency of the 6LiH 
neutron attenuator in experiments with the DD plasma. In 
these experiments the attenuation factor for the intensity 
of gamma-ray peaks due to inelastic neutron scattering 
reactions in the detectors was found to be ~100. At the 
same time the use of the attenuator reduces the intensity 
of gamma-rays in the range of >3 MeV only slightly (the 
attenuation factor is ~2), which indicates the high 
transparency of 6LiH to gamma-rays with energies of a 
few MeV. 
III. GAMMA-RAY SPECTROMETRY AT JET 
Effectiveness of gamma-ray diagnostic  techniques 
was demonstrated in the largest tokamak JET 
(EURATOM). At the moment gamma-ray diagnostics is 
one of the important techniques used for confined fast 
ions studied on JET.  
Gamma-ray energy spectra are measured on JET with 
three independent devices, one with a quasitangential, and 
two with vertical line of sight, through the plasma centre. 
Two of these devices are a calibrated BGO scintillation 
detectors with a diameter of 75mm and a height of 75mm. 
One of them is located in a well-shielded bunker, which 
views the plasma tangentially. The detector's line of sight 
lies in a horizontal plane about 30cm below the plasma 
magnetic axis. Another BGO-spectrometer is viewing the 
plasma vertically through the centre (R=2.95m). The 
gamma-ray spectra are continuously recorded in all JET 
discharges over the energy range 1-28MeV, with energy 
resolution of about 4% at 10MeV. The third device for the 
gamma-ray energy spectrum measurements is a NaI(Tl) 
scintillation detector with a diameter of 125 mm and a 
height of 125 mm, viewing the plasma vertically through 
the centre (R=2.95m). 
Recently, an upgrade project of  the JET gamma-ray 
spectrometer system has been undertaken.  This project 
concentrates on the development of  high rate, high 
resolution gamma-ray spectrometers to be installed at the 
JET. The main idea was to use large LaBr3(Ce) 
scintillators, which are just becoming available and offer 
a unique combination of energy resolution and 
scintillation decay time (Ref. 7). Another key feature is 
the use of digital data processing which will allow to 
handle count rates exceeding 1 MHz. 
Spatial profiles of the gamma-ray emission in the 
energy range Eγ > 1 MeV are measured in JET using the 
Gamma Cameras, which have ten horizontal and nine 
vertical collimated lines of sight. Each collimator 
corresponds to a poloidal-viewing extent at the centre of 
plasma of about 10cm. The detectors array is comprised 
of 19 CsI(Tl) photo-diodes (Ø20×15mm). The DAQ 
accommodates the gamma-ray count-rate measurement in 
four energy windows. This allows allocating specific 
gamma-ray peaks to be counted separately. 
A typical example of the tomographic reconstruction 
of the measured line-integrated profiles recorded during 
1s in a steady state phase of the plasma discharge is 
shown in Fig.1. It is seen clearly that the gamma-ray 
emission profile produced by fast D-ions (left-hand 
figure) differs from the profile from 4He-ions (right-hand 
figure). This effect can be explained by the difference in 
pitch-angle distribution between 4He beam-ions injected 
into the plasma quasitangentially and isotropic D-minority 
ions. The ability to separate gamma-rays with the Gamma 
Cameras from different energy bands allows the study of 
the fast ion behaviour in some important plasma scenarios.  
Fig. 1. Tomographic reconstructions of 4.44MeV γ-ray 
emission from the 9Be(α,nγ)12C reaction (right) and 
3.09MeV γ-ray emission from the 12C(d,pγ)13C reaction 
(left) deduced from simultaneously measured profiles 
(Ref. 3). 
IV. GAMMA-RAY DIAGNOSTICS IN ITER 
In ITER the gamma-ray diagnostics could perform 
the same functions as on JET. The main difference is a 
severe neutron/gamma background that requires the 
development of a special design of the sistem to 
maximize the signal-to-background noise ratio. For the 
fusion alpha-particle measurements the 2-D gamma 
cameras could be used, however, the gamma-ray detectors 
should be protected against harsh neutron emission with 
special neutron attenuators. That is why the results of the 
LiH-attenuator test on JET are very important for the 
development of the gamma-ray system in ITER. The 
Gamma Cameras could be integrated inside the radiation 
shielding of the radial and vertical neutron cameras, and, 
Chugunov et al.        GAMMA-RAY SPECTROMETRY FOR HOT PLASMAS
178 TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY        VOL. 59        JAN. 2011
which have the same type of the fan-shaped viewing 
geometry. It would be appropriate to have a separate line-
of-sight for each gamma-ray detector module with two 
adjustable collimators and intermediate flight tube, which 
would allow gamma-ray measurements over the wide 
dynamic range of ITER operation. The collimators could 
be individually rotatable to set an appropriate collimation 
and neutron/gamma suppression depending on 
experimental priorities. 
Fig.2. Possible arrangement of the gamma-ray 
spectrometer system in the channels of Radial and 
Vertical Neutron Cameras in ITER. 
The absence of vertical ports on ITER makes it much 
more complicated to develop the implementation of 
tomographic neutron and gamma-ray reconstruction 
systems. At the moment it is suggested to use a divertor 
port  for vertical viewpoint implementation.  
Strong magnetic field (order of 2T) in the divertor 
port makes it hardly possible to use conventional multi-
dynode photomultipliers as light detectors in vertical 
neutron and gamma detection systems, so it is suggested 
to use micro-channel plate photomultipliers instead. It has 
been carried out investigations of magnetic field impact 
on the performance of the gamma-spectrometer with the 
micro-channel photomultiplier used as a light detector. 
Recent tests of  micro-channel plates (MCP-PMT) gave 
encouraging results. LaBr3(Ce) detector with MCP-PMT 
has shown capability for spectrometric measurements in 
the 1.8T magnetic field (see Fig.3). The Ioffe cyclotron 
magnet was used in the tests as a source of magnetic field. 
In spite of worse energy resolution in comparison with  
conventional PMTs (by a factor of 2 approximately) 
MCP–PMTs give a chance to arrange the  tomographic 
measurements in ITER. 
0 400 800 1200 1600 2000
0
200
400
600
800
C
ou
nt
s 
pe
r 
ch
an
ne
l
Energy (keV)
Fig.3. Spectrum of radioactive source 22Na (511 and 1275 
keV gamma-lines) measured by LaBr3(Ce) detector with 
MCP-PMT in the magnetic field of 1.8T. 
V. CONCLUSIONS 
After 20 years of studies the gamma-ray spectrometry 
provides an effective tool for studies of fast ions behavior 
in the hot tokamak  plasmas. This method is unique 
nowadays, which can get information about density and 
energy distribution of alpha-particles in the thermonuclear 
tokamak plasmas.  Systematic studies of fast ions 
behavior at JET together with important technological and 
methodical developments of Ioffe Institute make a basis 
for realization of the gamma-ray spectrometric 
diagnostics in ITER.  
ACKNOWLEDGMENTS 
This work was partly fulfilled in the frameworks of 
“Bilateral agreement for Co-operation between the 
EURATOM and the Government of RF in the field of 
controlled nuclear fusion” and ISTC projects №№161-94, 
161-B96 and 3785p.  This report was supported by 
Rosatom (contract -N.4z.41.03.08.258). 
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3. V. G. Kiptily et al., Nucl. Fusion, 45, L21–L25 
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4. V. G. Kiptily et al., Rev. Sci. Instrum., 74, 1753 
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Chugunov et al.        GAMMA-RAY SPECTROMETRY FOR HOT PLASMAS
TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY        VOL. 59        JAN. 2011 179


GAMMA-RAY SPECTROMETRY OF HOT PLASMAS

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GAMMA-RAY SPECTROMETRY OF HOT PLASMAS

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