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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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