Abstract No. 74
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Detection of Industrial Americium Sources by an Innovative Gate
Monitor
Michael Iwatschenko
Thermo Fisher Scientific, Erlangen, Germany
In recent years several incidents have been reported, where orphan Am-241
sources, inadvertently included in scrap metal, were molten in the furnaces of
a steel plant. Despite the fact that the steel remains largely uncontaminated in
such melts (due to its physical properties most of the Am-241 activity ends up
in the slag and dust), the cost of clean-up and disposal are significant.
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Unlike other industrial sources (Co-60, Cs-137, Ir-192, Eu-152) and naturally
occurring radioactive materials (NORM), Am-241 predominantly emits low
energy gamma radiation below 100 keV (mainly 59 keV). The emission rate of
penetrating higher energy gamma radiation is very low (in the order of E-5), so
that even high activity sources can remain undetected by conventional
systems based on gamma detectors.
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In order to address this gap in the detection capabilities of conventional
industrial gate monitors, Thermo Fisher Scientific Messtechnik GmbH,
Erlangen, Germany has developed a highly sensitive neutron detector system
which can be integrated into existing Thermo Fisher gate monitors or can be
operated as stand-alone parallel installation. The design of the system was led
by the fact that
any Am-241 source emits as well a certain amount of neutron radiation. These
high energy neutrons are produced in the AmOxide source matrix by nuclear
alpha,n reaction. Compared to gamma radiation, these neutron particles are
highly penetrating and therefore much less attenuated by the shielding of the
source and the surrounding scrap material. In addition to the surprisingly good
detection capability for high activity Am-241 sources, these systems obviously
provide excellent protection against neutron sources (AmBe, Cf-252, Pu-238,
Pu-240, Cm isotopes) which are in common use in industry, including low
activity portable devices which may be deeply buried in dense scrap material
and remain undetected by conventional gate monitors.
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Detection of
Industrial Americium Sources
by an
Innovative Gate Monitor
Dr. Michael Iwatschenko
CETAS , May 20, 2015
[email protected]
The world leader in serving science
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The Americium Problem
In recent years several incidents have been reported,
where orphan Am-241 sources, inadvertently included in
scrap metal, were melted in the furnaces of a steel plant.
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Despite the fact that the steel remains largely
uncontaminated in such melts (due to its physical
properties most of the Am-241 activity ends up in the
slag), the cost of clean-up and disposal are significant.
Half-Life:
Radiotoxicity:
2
433 years
similar to Plutonium (Pu-239)
possible inhalation risk to slag pile
workers
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Wide-Spread Usage of Am-241 in the Industry
Source: IAEA-TECDOC-1459
A huge number of Am-241 gauges are/were in use
Î growing number of incidents expected due to much
longer half-life of Am-241 compared to lifetime of
the instruments…
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Reported Melting Events
Source: IAEA SSG-17
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Most probably the true
number of Am-241 melts
is much higher, since
the slag is not always
monitored for
radiation…
Am-241 Radiation Specifics
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Unlike other industrial sources (Co-60, Cs-137, Ir-192,
Eu-152) and naturally occurring radioactive materials
(NORM), Am-241 predominantly emits low energy gamma
radiation below 100 keV (mainly 14 keV and 59 keV),
which is easily shielded.
The emission rate of penetrating higher energy gamma
radiation is very low (in the order of E-5), so that even high
activity sources can remain undetected by conventional
gate monitors based on gamma detectors.
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Problem: Melting of Am-241 Sources in Steel Mills
Solution:
Timely Detection by (Secondary) Neutrons
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Am-241 Industrial Source Types
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Gauging devices using photon radiation
(14 keV X-ray and/or 59 keV gamma radiation)
Gauging devices using neutron radiation
Be-9 (alpha, n) C-12 + 4,44 MeV gamma
Typical activity range for both source types:
1 to 1000 GBq (app. 30 mCi to 30 Ci)
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Am-241 - Physics
662* - 722 keV 0.00055 %
Î app. 200,000 Gammas/s per Ci
but confusion with small Cs-137 source possible
significant self absorption in large Am-241 sources
Alpha:
Neutron:
5.5 MeV (100%) , short range
Unintended (often unknown)
Î app. 10000 n/s per Ci
via (Alpha, n) reaction in the AmO2 source
(O-17, O-18, and impurities e.g. F-19, Al-27)
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Gamma:
AmBe:
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Intended Neutron Source
Î app . 2 Mio n/s per Ci = 2000 n/s per mCi
Detection Limit (Bq) for “AmBe” is 200 times
lower than for “Am” sources
Neutron Detectors at Gate Monitors?
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Gate monitors at country borders have been equipped with
gamma & neutron detectors since the mid 1990‘s in order
to detect weapon grade Pu. Gate monitors at the entrance
of scrap yards and steel mills are usually equipped with
gamma detectors only.
Since app. 2008 conventional neutron detectors based on
He-3 proportional count tubes became unaffordable for
applications outside US-homeland security…
Today:
Li-6 doped ZnS(Ag) flat panel neutron detection technology
provides high neutron sensitivity at much lower cost than
He-3 detectors.
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New Flat Panel Neutron Detector Elements
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The detector operates by capturing thermal neutrons
through the Li-6 (n,alpha) reaction. This produces intense
scintillation pulses that are read out by a proprietary
combination of bulk wavelength shifting waveguide, solid
state photo multiplier and on-board electronics.
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FHT 1388S /SGS-2: New Neutron Module
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• Flat panel neutron detectors – insensitive to gamma radiation
• High Sensitivity: 160 cps per n/cm² s
Integrated component of FHT 1388 S gate monitor
or
stand-alone addition to any gate monitor…
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Ideal Measurement Location
Due to the very low background count rate of the neutron
detectors, the detection limit is much improved for longer
exposure time / slow speed:
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Î Measurement on stationary weighing bridge if possible!
Î Integration into exiting FHT 1388 S
or
Seperate installation
(which is preferrable over integration into any drive-through gate)
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Longer Useful Exposure Time for Neutron Detectors
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Gamma
Neutron
Gamma detectors receive radiation only during short time slot when vehicle
is passing by (few 100 ms)
Neutrons are much less absorbed in the steel load than gammas, i.e. can
travel longer distance in the load, undergoing multiple scattering events
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Test Measurements in Stainless Steel Scrap
3 Ci Am-241
in Tungsten shield
Detected by neutron radiation on BOTH sides of truck
(app. 3 respectively 10 times above alarm threshold)
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Sensitivity Test with 50 mCi AmBe
Provided by Courtesy of Jewo Metaal Rotterdam
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Direct Comparison (50 mCi heavily shielded AmBe)
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No Alarm!
Typical Gamma Gate Monitor
Alarm > 1000 %
FHT 1388 S neutron Module
Provided by Courtesy of Jwo Metaal Rotterdam
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Estimation of typical Detection Limits in Steel Scrap
AmBe:
Am-241:
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2 Detector System, 5 km/h:
app. 5 mCi
app. 1 Ci
8 Detector System,
on stationary weighing Bridge:
AmBe:
Am-241:
Note:
app. 1 mCi
app. 200 mCi
Data are based on center location of source Significantly better average detection results can be expected
for arbitrary location in the scrap load
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Additional Opportunities to detect Am-241 Orphan Sources
Shielding
Distance
Time
Instrument
gamma/neutron
Abandoned
Industrial Site
Source +
structural
material
Inspector’s
discretion
Inspector’s
discretion
Pocket Meter,
Hand held,
Backpack
Decommissioning
& loading
Source
Close (worker)
long
Pocket Meter
Gate & Border
Source +
load
2–3m
short
Gate monitor
Backpack
Grapple
Source +
some metal
0.5 - 2 m
medium
Grapple monitor
variable
long
Backpack
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Location
Piled up on yard Source +
some metal
Conveyor
Source
<1m
short
Conveyor belt
monitor
After
fragmentation
none
variable
variable
Pocket meter
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Double Layer Detection Strategy – Gamma & Neutron
Homeland Security
Industrial
Prevent Intrusion
Harbour
Airport
Bridge
Gate
Weighing Bridge
Ship Crane
2nd Line of Defence
Area monitoring
Coveyor belt, grapple,
area monitoring
Mobile Systems
(Vehicle, Backpack)
Mobile Systems
(Vehicle, Backpack)
Police officer with
radiation pagers
Worker with
radiation pagers
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Purpose
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Mobile High Sensitivity Gamma Neutron Screening
2 ea. flat panel
neutron detectors
in rear section of
backpack
1 ea. gamma
detector
in front section of
backpack
„Moving Detection Curtain“ on Site: FHT 1377 GN-2
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Routine Site Screening at no Extra Cost
Gamma/Neutron Backpack
installed in Site Security Vehicle
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Examples of Severe Incidents
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2 out of 6 listed most severe events
relate to neutron sources!
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Source: IAEA SSG-17
Neutron Monitoring Recommendation (2012)
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Page 27:
Neutron detectors are used to detect the presence of
fissile material such as 239Pu. These are frequently
used in border monitoring equipment where the
concern is with the detection of illegal transboundary
movement of nuclear material. There are, however,
neutron sources that are used in general industry.
Americium-241 in combination with beryllium, which
is used in moisture gauging equipment, is a common
example.
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The monitoring equipment should be selected in accordance
with the type of facility. Facilities that handle large consignments
of scrap metal should use stationary (portal) monitors for
monitoring photon (and sometimes also neutron) radiation
from consignments on arrival and any products (ingots, metal
bars, wastes, etc.) prior to dispatch. Such equipment should be
sufficiently sensitive to be able to detect small increments in the
level of radiation above natural background levels of radiation.
Monitoring of consignments on arrival facilitates the
identification of the origin of any radioactive material detected.
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For Further Information & Discussion
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Meet me at the
Thermo Fisher Scientific Stand
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tomorrow