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CEM 485-001 SPRING 2012
Nuclear Explosion Detection
Detection in an International Framework
Alexander T. Chemey
4/11/2012
Nuclear explosion detection presents staggering technical, political, and scientific challenges, all
of which are on a worldwide scale. The CTBT offers a policy framework for nuclear event
detection and international coordination. Non-nuclear methods are used to identify the blast
location and magnitude. Aerosol and Radioxenon detectors confirm or deny the nuclear nature
of the blast. Proof of the efficacy of such a system lies within the DPRK nuclear test of 2006.
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Introduction
The technology to develop and deliver nuclear weapons is intricate, to say the
least. When testing a new weapon design, or a new production method for an older
weapon, inquiries into reliability dictate the testing of these weapons. The strength and
policy implications of these weapons, have led to the creation of international
agreements for the reduction of nuclear weapons stores,1 anti-proliferation efforts,2 and
agencies to enforce these agreements.i A hallmark of these agreements, efforts, and
organizations is the enforcement of bans on nuclear weapons testing, bans that place
stringent logistical, legal, and technical strains on the signatories. The Comprehensive
Test-Ban Treaty3 banned nuclear weapons tests of any sort, provided for the creation of
an organization to ensure that party-states were in line with its intent,4 and created an
international framework for the cooperation of states in detecting nuclear weapons
tests.5 This framework uses a variety of nuclear and non-nuclear detector methods in
coordination to determine if a nuclear event did occur. Understanding both the
framework and the detector methods utilized by a CTBT is crucial for any
comprehensive look at nuclear explosion detection.
Through a combination of non-nuclear detectors, a location of a nuclear weapons
test can be identified, and the magnitude judged. Three modern examples of nuclear
weapons detectors, RASA, SPALAX, and SAUNA, offer insight into the present
generation of nuclear detectors. Radioactive Xenon (radioxenon) detectors focus on oft-
1 START, SALT, PTBT are three examples 2 The Nuclear Non-Proliferation Treaty is an example 3 CTBT; an agreement to ban nuclear weapons tests among the member states passed by the general assembly of the United Nations in 1996, though not yet in effect 4 The CTBT Organization 5 The International Monitoring System (IMS) and the International Data Centre coordinate with nuclear detector sites to ensure that communication between scientists and policy makers is never interrupted.
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produced fission products from nuclear weapons tests with significant advantages over
alternative detector objectives if the blast is underground or particularly weak. Current
research projects are investigating Germanium beta-gamma coincidence detectors for
applications in radioxenon detectors, which hold further advantages over present
radioxenon detectors. The case study of a small nuclear weapon test by North Korea in
2006 provides a meaningful example of the detector systems’ capabilities, in an
international framework.
Non-Nuclear Detectors and their Use
In its text, the CTBT delineates three methods of nuclear event detection beyond
“Radionuclide Monitoring.” Seismological monitoring, hydroacoustic monitoring, and
infrasound monitoring all are given as non-nuclear detection methods that will
ultimately form a large part of the IMS’s efforts to detect nuclear weapons tests
anywhere in the world.
In 1963, the Limited Test Ban Treaty (LTBT) banned nuclear weapons tests in
space, the atmosphere, and underwater. The only medium that was not banned by this
treaty was underground, primarily because of an uncertainty that such a ban could be
enforced by detection. ii Today, seismological science has progressed to the point where,
even with a small explosion, an underground nuclear weapons test can be detected by
the worldwide system of fifty seismological detector sites, or by the one hundred and
twenty secondary stations worldwide. Seismological monitoring, in the case of a
suspected underground nuclear explosion, is utilized to identify possible explosions via
seismic waves, as well as to identify the location and magnitude of the suspected blast. iii
Hydroacoustic monitoring and infrasound monitoring are both utilized in remarkably
similar manners to measure explosive pressure changes in water (hydroacoustic, see Fig
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1) and in the atmosphere (infrasound). iv All of these methods are only preliminary
detectors for the radionuclide testing that confirms or denies the suspected blast’s
nuclear characteristics, and thus the international response to the offender.
Figure 1v: Crossroads Baker test, first underwater nuclear test
Nuclear Detectors and their Use
It is only a nuclear detector, a detector that can identify radioactive particles or
products of radioactive decay, that can confirm or refute a suspected nuclear explosion’s
radioactive nature. The CTBT calls for sixteen nuclear laboratories supporting eighty
radionuclide stations spread across the world, with forty of them as radioxenon detector
sites.i Through the process of the International Noble Gas Experiment,6 four detector
systems were tested, all of which focused on radioxenon detection and analysis, albeit
through slightly different methods.iv Three of the INGE systems7 are presently being
used to supplement the gamma-ray aerosol detection systems which are in place at all
eighty locations. The examples of RASA (a gamma-ray aerosol detection system),vi
6 INGE; a world-wide experiment to determine which stations should receive noble gas monitoring capabilities, as well as test new designs of noble-gas based nuclear detectors. 7 SPALAX of France, SAUNA of Sweden, and ARIX of Russia are being used at present; the United States’s ARSA is not presently in use.
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SPALAX (a radioxenon high-resolution gamma spectrometry detection system),vii and
SAUNA (a radioxenon beta-gamma coincidence detection system) illustrate three
modern approaches to nuclear explosion detectors.
RASA
The United States Department of Energy’s Radionuclide Aerosol
Sampler/Analyzer (RASA, Fig 2) is the current8 aerosol detector at the Schauinsland
Monitoring Station (RN33, Southwest Germany), the location that was the testing site
for the INGE. The detector inside RASA is a high-resolution Germanium gamma-ray
detector. RASA compresses and then strains ambient air through a filter medium at high
speed, trapping 80% of the particles greater than .2 microns at operating conditions.viii
After a twenty-four hour delay for Radon to decay, the filter tapes are taken past the
ix
Germanium detector, producing a high-quality gamma-ray spectrum with very distinct
peaks.viii Detectors like RASA are deployed worldwide, sampling the atmosphere and
determining the nature of the background radiation for the region; these common
8 As of 2007.
Figure 2ix: A detailed diagram of RASA
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aerosol detectors that are complemented by radioxenon detectors for greater confidence
in the nuclear nature of an explosion.
Radioxenon Detection
What are the advantages of detecting radioxenon? There are many other fission
products, and some of them give off more radiation, but there are a few things that make
radioxenon exceptional for detection purposes. A partial table of isotopes radiated from
U-235 explosion is depicted in Table 1. Note that this table consists of initial percentages
of the fission products, and that all the isotopes present here decay, giving off noticeable
Table 1: A Partial table of
Isotopes and
isomers from U-235
warhead detonation
adapted from Li 1998x
signals. In the event of an underground explosion, the rock surrounding the device is
vaporized; molten rock then caves in the chamber. Elements with higher boiling points
do not escape this collapse, but more volatile elements can boil off and escape through
fissures in the rock, leaving detectable radioactive signatures for both themselves and
their daughter isotopes.vii Note the four isotopes of Xenon in Fig 3, all with mid-ranged
halflives. Consider as well the fact that Xenon has a very low boiling point and is a Noble
Gas. In Table 1, (from top to bottom) Zr through Nd are all non-volatile; Krypton has a
very short half-life, and decays (through Rb-89, also short-lived) into fifty-day half-lifed
Sr-89, which can be detected through RASA and other aerosol detectors like it. Xenon is
Isotope Yield (%) Half-Life Boiling Pt in C
Zr-95 6.299 64 d 4400 Mo-99 6.015 66.02 h 4600 Ru-103 3.65 39.4 d 3900 Ce-141 5.718 32.5 d 3400 Ce-144 4.872 284 d 3400 Nd-147 2.168 11 d 3100 Kr-89 4.33 3.18 m -150
Xe-133m 0.184 2.19 d -110 Xe-133 6.63 5.25 d -110 Xe-135 6.3 9.01 h -110 Xe-137 5.65 3.82 m -110
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non-reactive, volatile, has a variety of halflives of mid-length (allowing the drift of
radioactive materials to multiple detectors, and the measurement of isotopic/isomeric
ratios), and is produced in large quanitities, making it an excellent candidate for the
verification of an underground nuclear weapons test.vii For the forty stations dedicated
to radioxenon detection, there are multiple designs and methods to sample the
atmosphere and detect radioxenon from a nuclear explosion; SPALAX and SAUNA are
two present radioxenon detector system designs.
SPALAX and SAUNA
SPALAX (Fig 3) and SAUNA are remarkably similar in general design concept.
Both sample the air, use chemical and physical methods to remove pollutants from the
sample and concentrate the Xenon,9 and
then expose the Xenon sample to a detector
without cryogenic cooling needed at any
step in the process.xi,xii Arguably the largest
difference between these two detection
systems is the detector itself; SPALAX
utilizes a high-resolution gamma-ray
spectrometer,xi while SAUNA uses a plastic
scintillator and NaI detectors for a beta-
gamma coincidence system.xii
The IMS has a variety of benchmarks
for noble gas detector systems. Radioxenon
9 Both use activated charcoal and gas chromatography, among other similar aspects in this process.
Figure 3: A diagram of SPALAXxiii
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detectors must sample for no more than a full day before analysis begins, they must be
able to detect Xe-133 at concentrations of less than or equal to one millibecquerel per
cubic meter, and a strong emphasis is placed upon automation. Moreover, the Xe-131m,
-133m, -133, and -135 isotopes must all be detectable by either beta-gamma coincidence
or high-resolution gamma spectrometry. The expectation is that, if a typical IMS station
is a thousand kilometers away from a blast, the radioxenon produced by a one kiloton
nuclear explosion should be detectable with high confidence.xi
SPALAX’s gamma-ray spectrometer cleanly distinguishes between the peaks at
164, 81, 233, and 250 kev that are produced by the isotopes and isomers required by the
IMS for verification.xiii With only 17 kev of difference between the peaks for Xe-133 and
Xe-135, a high resolution detector is needed to get an accurate Xe-135/Xe-133 ratio.xi,iii
In Fig 4, the comparative gamma ray spectra for NaI(Ti) and HPGe(Li) detectors are
depicted. Compared to an NaI detector, the Germanium detector in SPALAX has very
Figure 4: gamma-ray spectrum as measured by a NaI and a Ge detectorxiv
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good energy resolution, practically meaning that SPALAX can cleanly distinguish
between the energy peaks in the air sample. As such, no secondary verification of the air
contents is required.
SAUNA takes a different tack on the issue. The SAUNA detector uses beta-
gamma coincidence detection to sample for radioxenon in the air sample. All four
isotopes that the IMS requires for radioxenon detectors have well-separated beta-
gamma decay modes with strong branching ratios in a convenient energy range. As a
coincident system needs both the beta- and gamma-decays in concert to be detected, the
use of a coincident detector system reduces background radiation, and provides an
accurate depiction of the Xenon isotope and isomer ratios.xii The SAUNA detector
(depicted in Fig 5) accomplishes this through the use of plastic scintillator cells and NaI
detectors arranged around the scintillator cells. The scintillators are designed to
Figure 5xv: SAUNA detector setup
stop the most energetic beta-particles given off,10 while the NaI detectors form a
gamma-ray spectrum.xv Previously discussed was the relative energy resolution of NaI
10 350 kev from the beta decay of Xe-133, also from Ringbom 2003
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and HPGe detectors. NaI detectors are more energy-efficient than Ge detectors; HPGe
detectors consistently have an efficiency of about a quarter of a percent of radiation over
a range up to 1400 kev, while NaI detectors have efficiencies ranging from one percent
at higher energies to six percent at lower energies.xv A higher energy efficiency means
that a smaller sample can be taken with the same amount of radiation detected, giving
NaI detectors an advantage over alternatives in a two-detector beta-gamma coincidence
system. With a higher efficiency and the beta-decay coincidence, SAUNA’s lower energy
resolution of the gamma decay resulting from the Xe isotopes and isomers does not
inherently make SPALAX superior; both are feasible options for nuclear explosion
detectors in an international system.
Germanium Beta-Gamma Coincidence Detectors in Radioxenon Systems
The ability to have high-resolution gamma spectra in a beta-gamma coincidence
detector would most likely improve the possibility of distinguishing between the Xenon
isotopes in a collected sample. Simultaneous measurement of the beta- and gamma-ray
energies would go a long way towards this goal. Each of the Xe isotopes emit a strong
branch x-ray in the 30-kev region11, and a detector that can cleanly distinguish between
these peaks would be a large step forward.xv When gamma radiation interacts with a
Germanium detector, electron clouds and holes are created; the electric field running
through the detector pulls apart these charge clouds towards the cathode and anode.iii
The signal is created by the collection of these charge clouds at the cathode and anode,
creating clean peaks at the energies of the photons that interact with the detectors.
High-Purity Planar Germanium Double Sided Strip Detectors provide a “best of both
11 Xe-135 is the weakest, with a 5% branch, while the other three emit a 50% branch.
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worlds” scenario, with excellent gamma and x-ray resolution (see Fig 6 for an example
of this with an isolated peak) augmented by good position resolution.xvi
Figure 6: GEANT simulation of 374 kev photon interacting in a Germanium detector; 1.7%FWHM
A logical step in nuclear explosion detection under a CTBT is the creation of a
new detection system. It would be similar to SPALAX and SAUNA in overall conception,
but would be a beta-gamma coincidence detector with a HPGe detector, rather than the
alternatives already in use. Combining the advantages of high energy resolution12 and
the greater tolerance to background radiation13 in a single detector for simultaneous
measurement of these particles’ energies would be a major step forward. The result of
such a system would likely be a smaller Minimum Detectable Concentration (MDC) of
radioxenon in the atmosphere, allowing a Ge-detector station to be farther away from
the source, for smaller air samples to be processed before analysis, and for the reports
generated to be transmitted to the IMS more quickly.
12 From the resolution of Ge detectors 13 From the coincidence detection
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Aerosol detectors remain on-station worldwide, to determine radiations
constantly. Modern operational radioxenon-specific detectors provide further and
particular insight into suspected nuclear events. Continued research into the
applications of Germanium-based detectors can provide new focus and sensitivity to
worldwide nuclear event detection. All of this theory is well and good, but the only
means of verifying an international system’s actual operative status is by an actual
nuclear event being confirmed or denied through the use of this international system.
Case Study: DPRK Weapons Test, 2006
On October 9, 2006, the DPRK14 claimed a successful nuclear weapons test.iii There was
no evidence for a space, water, or air blast. A 4.3 magnitude seismic event was recorded
at thirty-one stations across the world, indicating some sort of explosion had indeed
taken place within North Korea’s borders, localized by seismography at 41.294 degrees
North latitude and 129.094 degrees East longitude. This was a location that had been
observed by satellites in years prior, with the expectation that it could be a nuclear
testing location for North Korea. The seismic data indicated a blast between .4 and .8
kilotons of TNT, suggesting a nuclear blast or an extremely large conventional weapons
explosion that was meant to imitate a nuclear blast.xvii
Predictions for an underground nuclear weapons test were for 10-15 Bq of Xe-133
to be leaked over the first day, with somewhat less to seep out over the next three
days.xviii From October 11 to October 14 (two to five days after the explosion), a mobile
SAUNA II system was deployed to the nearest position possible within South Korea (see
Fig 7), to sample the atmosphere for radioxenon. (After the sampling, from November
until the following February, background radioxenon sampling was done, to identify any
14 Democratic People’s Republic of Korea, also known as North Korea throughout this paper
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errors that may have occurred due to other sources of radioxenon.15,xix) Sixty hours of
atmospheric sampling was done over these three days from the ROK (South Korean)
location. Meanwhile, because of a lack of noble gas detectors in the proximity (station
RUX58 was the nearest down-wind detector station, but it was not installed at that
Figure 7xx: Map of the event and SAUNA II in relation to the surrounding area
particular time), the nearest permanent and implemented INGE-approved detector was
7000 kilometers away, the SPALAX system at Yellowknife, Canada. Detector station
CAX16 utilized Xenon detectors to determine the nature of the nuclear blast, even from
afar.xx From the samples collected,16 augmented with a weather analysis process,17 and
15 There are a wide variety of anthropogenic background sources, which range from nuclear power plants to air liquefaction factories.xix
16 Taken from October 21 to 27
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using only a quarter of the planned noble gas stations, the very minor DPRK nuclear
event was detected. Had RUX58 been online, it is speculated that the single station
there would have provided ample evidence to the international community that a
nuclear blast had indeed been achieved by the DPRK. The data taken by the mobile
SAUNA II, when decay corrected, detected 7 x 1013 Bq at the release point, suggesting
that 0.7% of the Xenon from a 1kt nuclear blast had escaped.18,xx All indications, despite
the ad hoc and improvised nature of the detector arrangements, point to a miniscule
nuclear test by the DPRK.
A minor earthquake confirmed a blast in the hundreds-of-kilotons region at a
location within the DPRK. A radioxenon-based mobile detection system was set up to
observe the blast’s fallout. Despite the distance of 7000 kilometers, an INGE-approved
SPALAX system detected the small amount of radiation given off by the nuclear
weapons test when the atmospheric models predicted it would reach the detector. The
nuclear test by the DPRK is a useful case study in remote nuclear weapons detection.
Conclusion
There is much more to the detection of a nuclear event than any one technology
or model. Throughout this paper, reference has been made to a CTBT (as well as the
institutions described within its pages), but this is only an international framework that
provides a good example of a policy solution to a policy problem that requires nuclear
explosion detection. It should be noted that the ability to detect a nuclear blast is in no
way dependent upon a CTBT, but rather is wholly reliant on the existence of an
international infrastructure, one example of which is described in the CTBT of 1996.
17 “Weather backtracking.” Weather backtracking uses atmospheric data and weather patterns in coordination to determine the genesis of nuclear materials in the atmosphere. 18 1016 Bq of Xe-133 is expected from a 1kt nuclear blast; 0.7% is compatible with realistic scenarios
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To truly detect a nuclear explosion, non-nuclear methods are required to initially
detect and localize the blast. To determine the nature of particulate matter from an
explosion, gamma-ray aerosol detectors across the world confirm or refute suspected
nuclear explosions. Radioxenon testing provides secondary and focused confirmation,
specifically targeted at covert and small underground tests; although SPALAX and
SAUNA are satisfactory modern radioxenon detectors, Germanium beta-gamma
coincidence detectors offer more certainty still. Proof of the global detection network
capabilities is provided in the tiny underground weapons test by the DPRK in October of
2006. Even with threatened nuclear tests by the DPRK in the near future,xx one should
feel confident that any nuclear explosion will be detected by the systems in place today,
just as if a CTBT were in effect.
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