n-Tomography

Neutron tomography allows investigations of the interior of large samples (up to hundreds of cubic centimeters) with a spatial resolution of hundreds of micrometres. The neutron’s ability to transmit several centimetres of metal on the one hand and to be very sensitive to small amounts of light elements such as hydrogen, boron and lithium on the other makes neutron tomography a unique method for non-destructive testing in both industry and materials science. Some important applications of neutron radiography and tomography are investigations connected with quality tests of soot filters, adhesive joints, lubricate films and in-situ visualisations of water management in fuel cells. Interesting applications in archeology and medicine have also been reported recently.

The high potential of neutron tomography was the main motivation to setup a cold neutron imaging facility, CONRAD, at the Helmholtz Centre Berlin. The main requirements of the facility are flexibility and verstility, covering the scale from experiments demanding high flux such as real-time imaging and high-speed tomography to low-flux applications such as high-resolution and phase-contrast tomography. The cold neutron spectrum gives the advantage of increasing the image contrast due to the high-probability for neutron absorption.

Instrument description

CONRAD is positioned at the end of the curved neutron guide NL1b facing the cold source of the BER-II reactor. The existence of a neutron guide helps to achieve a high cold neutron flux in the order of ca. 2x108 n/cm2s at the end of the guide with a negligible background of gammas and fast neutrons. The facility was designed with two measuring positions. The first is located directly at the end of the neutron guide, where a high neutron flux is available, but with a poor beam collimation leading to an acceptable image resolution. The second is located at a distance of approximately 5 m from the guide end and uses a pin-hole geometry for higher image spatial resolution, but receives a lower neutron flux.

Instrument sketch


Instrument sketch

Instrument data

Spectrum

0.2 – 1.2 nm (Maxwellian distribution with a maximum at 0.35 nm)

Sample environment

Cooling water, air pressure, N-, He- and H – gas-lines

Measuring positions

Two (the parameters are listed below)

Detectors

CCD camera (Andor DW436N-BV) – 2048x2048 pixels (Position II)

CMOS camera (PCO 1200 hs) – 1024x1024 pixels (Position I)

Contact

N. Kardjilov

 

 

L/D

Resolution, µm

Max neutron flux at sample, n/cm2s

Exposure time, s

Beam size, cm2

Position I

~70

500-1000

2.0x108

0.01-0.5

3x10

Position II

 

 

 

 

 

- 3 cm pinhole

167

~300

2.4x107

1-5

12x12

- 2 cm pinhole

250

~200

1.6x107

5-15

11x11

- 1 cm pinhole

500

~200

5.8x106

10-25

10x10


Applications

High-resolution tomography

High resolution tomography experiments typically record 300-600 projections spaced equidistantly around 360°. The detector system has a geometrical resolution of about 0.03 mm. The total measuring time is 3 hours at L/D=250 and 6 hours at L/D=500.
High-resolution tomography sections of crack propagation in stainless steel crankshaft (diameter of 1cm) are presented as an example. The size of the crack is approximately 80 µm.

Fuel cell research

Neutron radiography is a powerful tool for visualization and quantification of the water distribution and transport in complicated devices like fuel cells. Neutrons provide a high sensitivity for the detection of water in a metal matrix. While the metal layers can be penetrated relatively easy, the probability for the interaction of a neutron with hydrogen and all hydrogen containing materials is high. That means that strongly different attenuation coefficients of the metal matrix and the water inside enable a high contrast even for small water quantities in, e.g., metal assemblies.

Imaging with polarized neutrons

The neutrons are sensitive to magnetic fields due to their magnetic moment, i.e. spin. Therefore as well as the conventional attenuation contrast image of a sample the magnetic field inside and around the sample can be visualized independently by detection of the polarization changes in the transmitted beam.
Polarized neutron radiography is based on the spatially resolved measurement of the final precession angles of a collimated and polarized monochromatic neutron beam that transmits a magnetic field, which is present inside and outside of a sample.
The potential of the method is demonstrated by visualization of the magnetic field around a (dipole) bar magnet.