Abstract No. 74

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. 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. 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. CE

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Dr. Michael IwatschenkoCETAS , May 20, 2015

michael.iwatschenko@thermofisher.com

Detection of Industrial Americium Sources

by an Innovative Gate Monitor

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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.

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: 433 yearsRadiotoxicity: similar to Plutonium (Pu-239)

possible inhalation risk to slag pile workersCETA

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Wide-Spread Usage of Am-241 in the Industry

A huge number of Am-241 gauges are/were in usegrowing number of incidents expected due to muchlonger half-life of Am-241 compared to lifetime ofthe instruments…

Source: IAEA-TECDOC-1459

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Reported Melting Events

Most probably the truenumber of Am-241 meltsis much higher, sincethe slag is not alwaysmonitored forradiation…

Source: IAEA SSG-17CETA

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Am-241 Radiation Specifics

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) NeutronsCE

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Am-241 Industrial Source Types

Gauging devices using photon radiation(14 keV X-ray and/or 59 keV gamma radiation)

Gauging devices using neutron radiationBe-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

Gamma: 662* - 722 keV 0.00055 % app. 200,000 Gammas/s per Ci

but confusion with small Cs-137 source possible significant self absorption in large Am-241 sources

Alpha: 5.5 MeV (100%) , short rangeNeutron: Unintended (often unknown)

app. 10000 n/s per Civia (Alpha, n) reaction in the AmO2 source(O-17, O-18, and impurities e.g. F-19, Al-27)

AmBe: Intended Neutron Sourceapp . 2 Mio n/s per Ci = 2000 n/s per mCi

Detection Limit (Bq) for “AmBe” is 200 times lower than for “Am” sources CE

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Neutron Detectors at Gate Monitors?

Gate monitors at country borders have been equipped withgamma & neutron detectors since the mid 1990‘s in order to detect weapon grade Pu. Gate monitors at the entranceof scrap yards and steel mills are usually equipped withgamma detectors only.Since app. 2008 conventional neutron detectors based on He-3 proportional count tubes became unaffordable forapplications 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

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

Integrated component of FHT 1388 S gate monitoror

stand-alone addition to any gate monitor…

• Flat panel neutron detectors – insensitive to gamma radiation• High Sensitivity: 160 cps per n/cm² s

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Ideal Measurement Location

Due to the very low background count rate of the neutrondetectors, the detection limit is much improved for longerexposure time / slow speed:

Measurement on stationary weighing bridge if possible!

Integration into exiting FHT 1388 S

or

Seperate installation(which is preferrable over integration into any drive-through gate)CE

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Longer Useful Exposure Time for Neutron Detectors

Gamma detectors receive radiation only during short time slot when vehicleis passing by (few 100 ms)

Neutrons are much less absorbed in the steel load than gammas, i.e. cantravel longer distance in the load, undergoing multiple scattering events

Gamma

Neutron

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Test Measurements in Stainless Steel Scrap

3 Ci Am-241in Tungsten shield

Detected by neutron radiation on BOTH sides of truck(app. 3 respectively 10 times above alarm threshold)CE

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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)

Provided by Courtesy of Jwo Metaal Rotterdam

Typical Gamma Gate Monitor

No Alarm!

FHT 1388 S neutron Module

Alarm > 1000 %

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Estimation of typical Detection Limits in Steel Scrap

2 Detector System, 5 km/h:

AmBe: app. 5 mCiAm-241: app. 1 Ci

8 Detector System, on stationary weighing Bridge:

AmBe: app. 1 mCiAm-241: app. 200 mCi

Note: Data are based on center location of source -Significantly better average detection results can be expectedfor arbitrary location in the scrap load

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Additional Opportunities to detect Am-241 Orphan Sources

Location Shielding Distance Time Instrumentgamma/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 – 3 m short Gate monitorBackpack

Grapple Source +some metal

0.5 - 2 m medium Grapple monitor

Piled up on yard Source +some metal

variable long Backpack

Conveyor Source < 1 m short Conveyor belt monitor

After fragmentation

none variable variable Pocket meterCETA

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Double Layer Detection Strategy – Gamma & Neutron

Purpose Homeland Security Industrial

Prevent Intrusion Harbour AirportBridge

GateWeighing BridgeShip Crane

2nd Line of Defence Area monitoring

Mobile Systems(Vehicle, Backpack)

Police officer withradiation pagers

Coveyor belt, grapple,area monitoring

Mobile Systems(Vehicle, Backpack)

Worker withradiation pagers

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Mobile High Sensitivity Gamma Neutron Screening

2 ea. flat panelneutron detectorsin rear section ofbackpack

1 ea. gammadetectorin front section ofbackpack

„Moving Detection Curtain“ on Site: FHT 1377 GN-2 CETA

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Routine Site Screening at no Extra Cost

Gamma/Neutron Backpackinstalled in Site Security Vehicle

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Examples of Severe Incidents

2 out of 6 listed most severe eventsrelate to neutron sources!

Source: IAEA SSG-17 CETA

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Neutron Monitoring Recommendation (2012)

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, whichis used in moisture gauging equipment, is a common example.

Page 28: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

Meet me at the

Thermo Fisher Scientific Stand

tomorrow

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