In efforts to improve the overall efficiency of present day irradiation
systems, a number of attempts have been made to better utilize the energies developed
in the systems. For example, in X-ray irradiation systems various schemes have
been developed to obtain a higher percentage of usefulness of the rays produced
by X-ray tube. That is, various systems have been developed in attempts to increase
the percentage of the energy converted to X-rays that is actually utilized to irradiate
a product or item. Further, various other systems and methods have been explored
to provide a more even distribution of the X-rays throughout the surface area of
the product being irradiated.
Also, in irradiation systems using gamma-quanta irradiation sources
such as cobalt-60 and cesium 137, various efforts have been made to provide more
even irradiation throughout the thickness of the product being irradiated. In prior
art systems, the absorbed energy distribution effective on the product being irradiated
depends on various factors including the material of the target, the distance of
the source to the target and on the geometry of the irradiation procedure. The
present invention provides a unique system and method for obtaining a means for
improving the efficiency of the desired radiation.
The system and method of the invention utilize a source of X-ray or
gamma-ray irradiation which is directed to irradiate a product. The rays penetrate
the product, and significant amounts of radiation (rays) exit the product on the
opposite surface of the product. A radiation reflective low Z (atomic number),
high density material is provided to reflect the rays penetrating the product.
The reflected rays are directed and reflected back to the product to again irradiate
the product thereby utilizing the reflected rays to provide a "secondary" irradiation
source to effectively "re-irradiate" the product.
The present invention improves the methodology and structure of irradiation
systems by utilizing the principal that, in many irradiation procedures, the irradiation
provided to the product penetrates that product and there is a significant amount
of photons which penetrate and exit the product.
The present invention reuses the photons which have passed through
the irradiated product and exited the product. These exiting photons are reflected
back to the product to re-irradiate the product to thereby provide more efficient
This utilizes radiation exiting the product, which has heretofore
been wasted, to re-irradiate the product.
The invention can also provide a more even distribution of an absorbed
dose throughout the surface area of the product being irradiated and throughout
the thickness of the product.
The use of unique irradiation techniques can provide an improved
irradiation system and method.
The foregoing features and advantages of the present invention will
be apparent from the following more particular description of an example in accordance
with the invention. The accompanying drawings, listed herein below, are useful
in explaining the invention. The drawings show:
- Fig. 1 shows a preferred embodiment of the inventive system and method wherein
a reflective, low Z high density material reflects the radiation (X-rays), which
exit the product, back to the product to re-irradiate the product;
- Fig. 1A is a cross-section of a modification of the embodiment of Fig. 1 wherein
the sides of the reflector are formed to be vertical (as oriented in Fig. 1);
- Fig. 2 is a table showing the percentage increase in the dosage provided by
the invention when tested with a water equivalent phantom;
- Fig. 3 is a graph showing the percentage increase in the dose provided by the
inventive method and structure; and,
- Fig. 4 is a table, essentially an extension of the table of Fig. 2 showing
the dose distribution in a standard water equivalent phantom positioned between
a 160 kV tube and a boron carbide reflector.
The principal of the invention relates to all types of electromagnetic
radiation, i.e. electronically produced X-ray and also gamma-quanta emitted after
radioactive decay of naturally radioactive isotopes like Cesium-137, Cobalt-57,
Cobalt-60 and all other X-ray emitters. The inventive system is constructed such
that the irradiation interacts with the low Z material to obtain as much back scattered
radiation as feasible, and with as little absorption of the radiation as practical.
The reflector used in the inventive system may be any low Z, high density material;
in the various embodiments of the invention, boron carbide, boron and carbon have
been used since these three materials appear to be the best for the purpose of
the invention. In the embodiment of the invention depicted in Fig. 1, relatively
thick (bulk) material from one inch to several inches (2.5 to several cm) in thickness
is used as the reflector in order to utilize the entire spectrum, i.e. all energies
up to 160keV. In the embodiment where mono-energy gamma-quanta from radioactive
decay is used the thickness of the reflector can be precisely calculated to obtain
In the embodiment of the inventive system and method as depicted in
Fig. 1, an X-ray tube 21 of any suitable known type provides X-rays 22 to irradiate
a product/object 23. As described above, the invention is applicable to the other
type of irradiation as described above, the principal of the invention is to effectively
reuse the photons produced by a source to re-irradiate the product to thereby provide
more efficient irradiation system and process. That is, the invention is applicable
to various sources of the electromagnetic radiation. The description of the embodiment
of Fig. 1 is thus generally inclusive for the other sources mentioned.
Referring still to Fig. 1, X-rays 22 are directed to enter (penetrate)
the upper surface (as oriented in the drawing) of the product. A portion of the
radiation (rays), indicated at 22A, penetrates and exits the product at the opposite
or lower surface of product 23. Also, as will be appreciated some of the X-rays
also exit at the sides of the product. A radiation reflector 24, comprising a low
Z (atomic number), high density material such as boron carbide, boron or carbon
is positioned to reflect a major portion of the radiation 22B exiting the product
23 back to irradiate the product, effectively from the bottom upwardly.
Note that the term, "high density material" referred to herein, comprises
boron, boron carbide, carbon or the like wherein the density is about 2 to 2.5
g cm3. These materials have the highest density amongst the low Z chemical
elements. A low Z material is chosen because of lower absorption of the irradiating
rays. It is known from physics that the absorption of X-rays and gamma-quanta rises
as Z to the 5th power and diminishes by energy as E to the 3.5 power where Z is
the atomic number of the absorber and E is the energy of the photons. This means
that the low energy photons like X-rays or gamma rays would be highly absorbed
by high Z materials. The best absorbers are high Z chemical elements and the best
scattering materials, i.e., material with low absorption capability are low Z chemical
elements. It is an additional feature of the high density material used that it
diminishes the depth of penetration into the reflector material layer thereby
permitting the thickness of the reflecting layer to be decreased. The reflector
24 can comprise a planar surface, and/or the reflector 24 may be contoured to better
direct the reflected X-rays back to the product, as depicted in Fig. 1. The reflector
should be at least three quarters (3/4)of an inch in thickness, and in the embodiment
described with relation to Figs.1, the reflector is 10 cm in thickness (2.54cms
equals about 1inch; 1.90cm equals about 3/4 inch).
Reflectors of boron carbide , boron and carbon have been used in the
inventive system. In one embodiment, boron carbide is used as the material for
the reflector 24 since it is readily available in the marketplace. All three materials
mentioned provide excellent results as a reflector of irradiation rays. Importantly,
all three materials are quite stable and will not deteriorate with use. Stated
in another way, all three materials can withstand the bombardment of the radiation
without any substantial alteration in their photon-reflective characteristics.
A comparison was made of the outputs of reflectors made from each
of the mentioned materials, i.e., and it has been found that the outputs from a
pure boron reflector as well as from a carbon reflector follow essentially the
output curves [a]of boron carbide. The boron and carbon reflectors actually provide
slightly higher peak outputs at the lower energy levels with carbon providing the
highest peak outputs. However, as mentioned above boron carbide is used in the
embodiment shown because it is generally available, durable and practical. Boron
carbide has the highest density (2.52) amongst the three materials noted herein.
In the embodiment of Fig. I, the reflector 24 has its sides or ends
25 angled upwardly, such that the reflected beam is directed to the bottom surface
of the product 23, and also to the sides of the product to provide a more uniform
irradiation to the entire product. It should be understood that the reflector 24
can be configured to accommodate products of different sizes and shapes. As depicted
in Fig.1A, if the product is circular, the reflector 24 can be configured to have
a circular recess 26 and vertical sides 25A of selected thickness, to receive the
product and more evenly reflect and re-irradiate its bottom, sides and even the
top surface. In the case of electronically produced X-rays, the thickness of the
reflector is chosen to effectively reflect the high energy in the broad X-ray spectrum.
In the case of a gamma-ray source, it is easier to determine the proper thickness
of the reflector, because the thickness can be adjusted (tuned) to only one energy.
Figs. 2 and 3 show the results of tests conducted to quantify the
improvement provided by the inventive method and apparatus. The test set-up was
modeled to obtain results over a wide band of voltages, i.e., for commercially
useful types of systems. It is, of course, known that water is a standard by which
useful X-ray irradiation can be measured, particularly when considering irradiation
of blood transfusion bags or containers, meat food products and vegetables.
The analysis to be described in connection with Fig. 2 and Fig. 3
was on a system such as shown in Fig. 1. Specifically, a four (4) cm thick water
equivalent phantom 23 comprising water equivalent polystyrene layers was positioned
to receive the radiation provided from the tungsten anode of the X-ray tube 21.
The results shown in Fig. 2, were obtained when the product was positioned 10 inches
(25.4 cms) from the output port of tube 21. The layers were located between the
X-ray tube and the reflector 24 comprised a 10 cm thick flat boron carbide member.
A standard aluminum or copper filter, not shown, filtered the X-rays from X-ray
In Fig. 1, for purposes of depiction of the X-rays 22 penetrating
the product 23 and the depiction of the reflected X-rays 22A, the space between
the product 23 and reflector 24 has been exaggerated. Preferably, the upper surface
of reflector 24 is placed in a position closely adjacent the bottom surface of
the product. For example, when the product is mounted on a conveyor belt, the reflector
is mounted immediately below the belt. The test results obtained in Figs. 2-4,
were obtained with the upper surface of the reflector 24 in position essentially
abutting the bottom of the phantom product.
For the comparisons indicated in Figs. 2 and 3, the system was first
operated without the reflector 24 and readings taken of the data obtained. Next,
the reflector 24 was mounted in the system and readings taken of this data. Fig.
2 is a table showing the dose distribution in the 4cm thick water equivalent phantom
product (standard layered phantom comprising suitable layers of plastic) irradiated
by a 160 kV X-ray tube. It was found that the dose distribution decreases almost
linearly from the top surface to the bottom surface of the phantom. Without a reflector
24 and assigning the value of 100% to the dosage at the top surface, it was found
that the dosage at the middle (at the 2cm thickness) of the phantom was 76% of
the dosage at the top of the phantom, and the dosage at the bottom surface was
49%. With the boron carbide reflector 24 placed in position in accordance with
the invention as indicated in Fig. 1, the dosage at the middle of the phantom was
90% percent of the dosage at the top surface, and the dosage at the bottom surface
was 70% of the dosage at the top surface. That is, the dosage distribution was
improved by about 14% at the middle of the phantom and 21% at the bottom of the
phantom. The table of Fig. 2 shows the actual increase in the dosage (when
using a 160 kV tube) as a result of providing the reflector 24.
The graph of Fig. 3 shows the results of calculations indicating
the percentage increase as X-ray sources operating at higher kV's are used.
As is known, X-ray tube sources are used in the 160-300 kV range; above 300kV,
electron accelerators are used as the source. In the graph of Fig. 3, the axis
of abscissas indicates the kV (voltages) of respective X-ray sources having accelerating
voltages varying from 160 kV to 10 MV. The axis of ordinates shows the dose increase
in percentage. At a voltage of 160 kV, the percentage increase is about 72.5%;
at 300 kV the percentage increase is about 42.5%; and at 1MV the increase is about
37.5%. Note that the percentage of increase is calculated to remain essentially
constant from 1MV to 10 MV.
The table of Fig. 4 shows the dose distribution in a 4-cm water phantom
positioned between a 160 kV X-ray tube and a boron carbide reflector. The table
shows that, for the system depicted, the reflector compensates for distance variation
between the tube and the product. Note that the dose ratio (dose at the top surface/-dose
at the bottom surface of the product) remains quite level for the distances from
8 inches to 16 inches. In the prior art systems, i.e., systems not using the inventive
reflector system, the effective dosage varies as the reciprocal of the square of
the distance between the product and the source. Thus a significant feature and
an advantage of the inventive system and method are that it compensates for the
influence of the increase in distance by between the source and the product by
using a radiation reflector. As mentioned above, in prior art systems the effective
dosage varies as the reciprocal of the square of the distance (1/r2
) i.e., low r- high dose, and high r-low dose; where "r" is the distance between
the source and the item or product being irradiated. This of course means that
in prior art systems, when the product being irradiated is positioned close to
the source, the surface of the product closest to the source receives quite different
amounts of radiation than the surface of the product farthest from the source i.e.,
the uniformity of irradiation becomes worse as the distance between the source
and the item is decreased. It can be readily appreciated that in prior art systems,
the dose uniformity between the top and bottom surfaces of a product is better
when the product is positioned at a greater distance than when it is positioned
closely to the source, as can be readily determined mathematically. In such prior
art systems there are several variables which may not vary linearly. For example
as mentioned, the effective dosage varies as the square of the distance between
the tube and the product, the thickness of the product affects the dosage. In contrast
to the prior art, in the inventive system the product can be positioned closer
to the source and get more radiation in the entire volume of the product without
worsening the top/bottom ratio; this is due to the fact that the
reflector provides a compensating factor in that the reflector helps the bottom
surface obtain more absorbed dose. In the inventive system, the thickness of the
reflector also affects the dosage but can also provide a compensating factor.
The specific data shown in the tables of Figs. 2 and 4 may vary somewhat
for different voltages, distances, and the thicknesses of the product and reflector.
However, calculations indicate the compensating effect indicated in the table is
generally applicable when a reflector in accordance with the invention is utilized.
It should be understood that the inventive system applies to reflectors using any
low Z, high density material although boron, boron carbide and carbon are the best
materials to use for the inventive purposes.
An important advantage provided by the inventive system and method
is that the product is more uniformly irradiated throughout the thickness
of the product. Further, the inventive system provides a more even irradiation
throughout the surface area of the product, i.e., the inventive system equalizes
the doses absorbed by the central area of the product surface and the doses absorbed
by the peripheral area of the surface which may be at different distances from
the source (see Fig. 1). Federal regulations require that the surface of the product
that is farthest away from the ray source be irradiated within a certain range
of the irradiation effective at the surface of the product closest to the ray source.
The basis for this requirement is that the irradiation applied to various products
must be effective to fully penetrate the thickness of the product, and must provide
a uniform dose, within prescribed ranges, throughout the thickness of the product.
In compliance with these regulations, the inventive system and method provide irradiation
to the product from multiple sides by using a unique system and method comprising
a single source of radiation and a radiation reflector which provides a more uniform
dose to the product, i.e. it tends to equalize and balance the irradiation of the
product from a single ray source throughout the surface area and thickness of
the product. At present, certain prior art equipment includes two X-ray sources
for irradiating a blood transfusion bag. By utilizing the present unique inventive
scheme, the same equipment can use one X-ray source with a reflector, rather than
two X-ray sources; the advantages are obvious.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood by those skilled
in the art that various changes in form and details may be made therein without
departing from the scope of the invention.