In the bread and bakery industry metal detectors are used throughout the process in order to remove metal fragments as soon as they are introduced.  Metal fragments or contaminates can be introduced in to the product stream through a number of ways.  The most obvious is within the processing of raw ingredients in to the final goods.  Here where mixing machines used to combine dry raw ingredients such as sugar and flour stir them together with large steel mixing blades.  These blades can fatigue over time and small pieces can break off and find there way in to the product.  Many bakeries will install an inline metal detector just after mixing to capture and reject automatically the metal fragments.  

Dough forming usually takes places just prior to entering the ovens.  The dough either enters a rounding machine that creates round dough balls for the production of buns or loaves of bread.  For bread products the dough ball typically passes under a metal chain hanging over the conveyor belt.  This chain slows the top of the dough ball down enough to cause it to roll over itself creating a long round dough ball.  This is then placed in to the pan as it enters in to the oven.  This chain is another spot where metal can find its way in to the product.

After baking the buns or loaves are removed from there baking pans and cooled off on a long circular conveyor belt.  This belt system is a chain link steel conveyor belt and can introduce metal in to the product.

Metal detectors are commonly placed after the cooling racks and just before the slicing machine.  This is an ideal location for the metal detector to capture metal fragments.  However, this is not the last location for the possible introduction of metal in to the product.  The slicers are long steel blades made from thin stainless steel.  Most bread manufacturers fail to install metal detection equipment after the slicer and run the risk of having contaminated product.

Industrial metal detection equipment is a critical piece in the quality control process of any bakery plant.  Location and correct operation of these devices is critical to ensuring the end product is contaminate free.

Absorption in water.

In the graphs, :/D is the mass attenuation coefficient, and :en/D is the mass energyabsorption coefficient (defined in ICRU report 33, 1980). The graphs have been taken from NIST Physics Laboratory, Physical Reference Data (See http://Physics.nist.gov/PhysRefData).

If we consider a solid material, like glass lead, we can observe that there are points of

discontinuity because, at certain energies, x-rays match the energy of the orbital of the

atoms.

Absorption in glass lead.

To a first approximation, the mass attenuation coefficient varies as the third power of the atomic number of the absorber. (See X-Ray Fluorescence Spectrometry Second Edition, Ron Jenkins, edited by JohnWiley & Sons, Inc, section 1.1).

It is not our intention to go any further into this subject. We only wanted to show that considerations about the nature of the materials are essential in the design of an x-ray inspection machine. This is true for every part of the machine that can be exposed to x-rays, either in an active or in a passive mode.

Principles of X-Rays

X-Ray definition, history.

Wilhelm Conrad Röntgen discovered x-rays at the end of the 19th century in Germany. X-rays are electromagnetic radiations produced by the deceleration of charged particles (normally electrons) or by the transition of electrons in atoms from one orbital level to another. The first method (deceleration of electrons) is the one used in the applications that are subject of this document.

The wavelengths of x-rays range from 10-8 m to 10-12 m, with corresponding frequencies of 1016 to 1021 Hz.

The energy of electromagnetic waves (such as gamma rays, ultraviolet, visible light, infrared, radio waves, etc.) is related to their frequency by the following formula:

E = h < [01]

Where E is the energy associated with the wave, h is Plank’s constant, and < is the frequency of the wave. This formula simply shows that the energy of an electromagnetic wave increases proportionally to the frequency. 

Wavelength and frequency of electromagnetic waves are related each other by the formula:

c = 8 < [02]

Where c is the speed of light in vacuum (equal to 3 108 m/s) 8 is the wavelength in m, and < is the frequency in Hz. This formula shows that an electromagnetic wave with a certain energy E can only have one wavelength 8 equal to: 

8 = c / < = h c / E [03]

Where h and c are constants. In particular, the wavelength of an x-ray decreases as the energy increases. This explains why higher energy x-rays penetrate better into matter. The matter is made of atoms with their electrons and nucleus, and of a lot of empty space. Denser matter has less empty space. To penetrate the matter without being absorbed, an x-ray has to find its way through the empty space without interacting with subatomic particles. In very simple terms, a wave with a smaller wavelength has a higher probability of passing through subatomic spaces without interaction.