WO1990006581A1

WO1990006581A1 - Structure for shielding radioactive radiation

Info

Publication number: WO1990006581A1
Application number: PCT/HU1989/000056
Authority: WO
Inventors: Péter Teleki
Original assignee: Teleki Peter
Priority date: 1988-11-28
Filing date: 1989-11-24
Publication date: 1990-06-14
Also published as: AU4528789A; EP0400121A1; CA2004079A1

Abstract

The invention refers to a structure for shielding radioactive radiation, the structure having first and second sides, the radioactive radiation (R) striking the structure on the first side, the structure comprising at least three structural layers (1, 3, 5, 11), wherein each of the first two structural layers, taken in sequence from the first side toward the second side, comprises an element which converts at least a part of a first kind of radioactive radiation into a second kind thereof, and it comprises at least two titanium structural layers (1) and a middle part arranged between the titanium structural layers (1), wherein one of the titanium structural layers (1) constitutes the first side and the middle part includes at least one intermediate structural layer (5) made of beryllium and/or magnesium.

Description

STRUCTURE FOR SHIELDING RADIOACTIVE RADIATION

FIELD OF THE INVENTION

The present invention refers to the field of the pro¬ tection against radioactivity and offers a structure for shielding radioactive radiation especially under very speci¬ fic conditions, e.g. in the cosmic space. Hence, the object of the present invention is a structure for shielding radio- active radiation, having first and second sides, the radio¬ active radiation striking the structure from first side, the structure comprising at least three structural layers and each of the first two structural layers, taken in sequence from the first side, comprises an element which converts at least a part of a first kind of radioactive radiation into a second kind thereof.

BACKGROUND OF THE INVENTION

The protection of the human beings and goods against radioactive radiation is generally secured by specific ma¬ terials described in detail in the literature of the art. The protection systems are realized by the use of specified material structures offering security in the presence of a source of a well-defined kind of radiation. In the normal conditions, all nuclear and X-ray sources applied in proces¬ ses controlled by the society are well-defined and therefore the existing methods of protecting the environment against the possible damages caused by this kinds of sources are sa¬ tisfactory.

From the related art, see e.g. in US-PS 2 580 360 (granted in 1945 to Ph. Morrison), US-PS 4 081 689 (granted in 1978 to Reiss) or GB-PS 2 117 964 (granted in 1982 to Amershem Int. pic) follows that the philosophy of protection against radioactive radiation has been long time based on systems consisting heavy metals, especially uranium and/or lead. The proposed shield structures are expensive and dif¬ ficult to move because of the weight. Therefore an important task should be seen in reducing the thickness and weight of such kinds of shields.

A new approach to the problem of the structures for shielding radioactive radiation can be found in the US-PS 4795 654 (granted in January 1989 to P. Teleki). The pro¬ posed structure secures shielding X-ray and gamma-radiation. It comprises a three layer system composed of different ele¬ ments, which converts the X-ray and gamma-radiation into an- other kind of radiation, on absorbing energy of the radia¬ tion. The first layer forming the first side of the struc¬ ture for receiving the radiation - the radioactive radiation strikes it - consists of uranium, gold, lead, osmium, rhe¬ nium, tungsten and/or tantalum. The second layer arranged behind the first is consisted of tin, indium, palladium, rhodium, rutheniu , molybdenum, and/or niobium. The third, i.e. the rear layer may be made of zinc, copper, nickel, co¬ balt, iron, manganese, chromium, vanadium and/or titanium. Each of the layers can be consisted of one element or a mix- ture (alloy) including at least two of the materials listed up above. The structure proposed by the US-PS 4795654 is made of relatively heavy metals and offers no protection against neutrons and alpha-particles. The problem of safety under conditions of intensive neutron radiation follows from the fact that the high energy neutron flux impacting the plurality of materials results in generating high-energy gamma radiation causing also difficulties in securing the protection. The plurality of metals listed up in the speci¬ fication creates the possibility of realizing effective pro- tective structures in environment comprising defined, i.e. known and localised sources of X-ray and gamma-radiation up to a middle energy level.

The known radioactive shield structures described in the art offer no satisfactory solution when high security protection is to be ensured in a radioactive environment ge¬ nerated by sources difficult to identify. The situation of such kind can occur especially in the space technology. In the cosmic space uncontrolled radiation sources act having sometimes intensity changing in wide range which may cause heavy damages to the apparatuses applied in the cosmic ve¬ hicles and laboratories. In war and catastrophic situations the same problem may arise also on the Earth.

In the environment of the cosmic space the most daπ- gereous are the particle showers comprising neutral and charged particles together with the high energy X-ray and gamma-radiation. The high energy elementary and charged par¬ ticles and radiation are capable of inducing different kinds of atomic reactions in the materials of the apparatuses app¬ lied and in the shield structures prepared generally in form of plate like elements (armour plates) . The existing power limitations of the rocket vehicles in the space technology do not allow to apply heavy and thick shields and therefore it is practically impossible to avoid many damages caused to the means of this technology applied for exploring the cos- mic space and/or realizing specific technologies in the con¬ ditions of the weightlessness.

SUMMARY OF THE INVENTION

Hence, the object of the present invention is to create a relatively light but effective shield structure applicable on aeroplanes and in aerospace engineering means securing high safety level protection against different kinds of the radioactive radiation and capable of securing the required safety up to the energy range 50 MeV character- izing the most intensive particle showers.

The invention is based on the following considera¬ tions.

The high energy radiation is always capable of acti- vating the elements forming any shield. In this activation process often occurs that the basic material becomes also radioactive. The activation process can not be avoided at all and therefore the solution should be found in the appro¬ priate material structure of a shield. The strongest influence can obviously be experienced on the first side of the shield which receives the radio¬ active radiation. This side subject also to optical and inf¬ rared radiation - the last can generally be compensated without dificculties by the known means. Of course, when se- lecting the thickness of the layer on the first side, it is important to take into account that a vaporization process can take place under influence of the kinds of radiation differing from the radioactive ones.

The electroπes falling into the first side can generate bremsstrahlung in the X-ray range. The alpha-par¬ ticles can cause nuclear reactions only in the light ele¬ ments, having atomic number above 20. The slowing down pro¬ cess of the electrones results in the requirement that the first side shouldn't include elements having atomic numbers exceeding 60. This is the range of the elements with middle atomic numbers. As for protecting against proton showers it is also advantageous to apply even this atomic number range. In the first side neither the very light nor the heavy ele¬ ments are advantageous. The measurements and analysis show that the maximal energy of the gamma- and neutron radiation to be expected in normal conditions doesn't exceed 20 MeV. In specific conditions 50 MeV can be expected also. This is, however, the maximal range to be taken into account because over 50 MeV another mechanisms play role. In the shield the reaction (gamma, n) should be realised under influence of the gamma-radiation and the neutron radiation has to cause a change in the atomic number +1. Of course, other reactions can take place, too. Here it is important to select the re¬ actions according to their probability and effective cross- -section.

On the basis of the above consideration a material structure should be realised wherein the elements resulting from the transformation reactions of atomic nuclei form also a metal which can coexist with the basic material and gives sufficient mechanical strength. Therefore it is important to select the structure according to the long term changes of the nuclei, i.e. to make a choice on the basis of the worst expected characteristics. The investigations show that no element can bear longer series of loss or capture of πeut- rons and the only one element resulting in another elements of sufficient strength after the loss of one neutron or a series of captures of neutrons is the titanium, Ti.

According to the recognition recapitulated above the titanium as structural layer is also very effective in com- biπation with a light metal, especially magnesium and beryl¬ lium when protection should be given against showers of charged particles, especially alpha-particles.

Thus, the present invention proposes a structure for shielding radioactive radiation, the structure having first and second sides, the radioactive radiation striking the structure on the first side, the structure comprising at least three structural layers, wherein each of the first two structural layers, taken in sequence from the first side to¬ ward the second side, comprises an element which converts at least a part of a first kind of radioactive radiation into a second kind thereof. The structure comprises at least two titanium structural layers and a middle part arranged be¬ tween the titanium structural layers, wherein one of the ti¬ tanium structural layers constitutes the first side and the middle part includes at least one intermediate structural layer made of beryllium and/or magnesium.

It is advantageous to apply in the proposed structure the intermediate structural layer comprising beryllium and separated from the titanium structural layers by shield structural layers prepared with boron for slowing down neut¬ rons.

In another advantageous embodiment, of the proposed structure the intermediate structural layer is made of be¬ ryllium dispergated in form of beryllium oxide in magnesium and/or copper.

In a preferred embodiment of the structure proposed by the invention the shield structural layers consist of boron dispergated in magnesium.

According to another possibility the shield struc- tural layers consist of filaments made of a boron or gra¬ phite, the filaments are covered with a layer made respec¬ tively of graphite or boron.

In a further preferred embodiment of the proposed structure the structural layers are separated by a layer including at least one oxide, nitride or carbide of boron or titanium.

It is also preferred if the proposed structure com¬ prises an outer covering layer consisted of titanium-nitride and/or rhodium, wherein the outer covering layer is prepared on the first side.

The protection safety is especially high when the structure realised according to the invention comprises at least thirty-two structural layers of preferred thicknesses not exceeding 0.01 mm, the structural layers forming a body of thickness at least about 0.3 mm, wherein the structural layers are separated by compound layers of thicknesses at most 0.001 mm. The compound layers generally consist of at least one oxide, nitride or carbide of at least one metal selected from boron, magnesium and titanium. The protection safety offered by the structure pro- posed by the invention can be further improved by applying a graphite sheet covering the first side.

The structure according to the invention secures pro¬ tection against showers of neutral and charged particles, and gamma- and X-ray radiation, as well. Of course, the me¬ tal layers are capable of capturing beta-radiation without any damage.

BRIEF DESCRIPTION OF THE DRAWINGS

The shield structure proposed by the invention will be further shown in more detail with reference to the at¬ tached drawings and several embodiments described by way of example only. In the drawings FIG. 1 illustrates a cross-section of an eight-layer structure with some separating compound la¬ yers, FIG. 2 shows a cross-section of a structure compris¬ ing a high number of, e.g. thirty-two layers, and

FIG. 3 is the cross-section of a relatively simple preferred embodiment of the structure const¬ ructed according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure proposed by the invention (Fig. 1) con¬ sists of at least two titanium structural layers 1 and a middle part therebetween. The middle part is consisted of either an intermediate structural layer 5 limited by two shield structural layers 3 lying at the titanium structural layers 1 or a magnesium intermediate structural layer 11. The structural layers form a body with preferred thickness at least 0.3 mm. The structure is arranged in the way of propagation of a kind of radioactive radiation R, impacting a first side of the structure, and this side is equipped with an outer covering layer 7 made of titanium-nitride, rhodium (_Λ5RI" or graphite. The shield structural layers 3 are made of bor- on (,-B) and the intermediate structural layer 5 consists of beryllium Be) . The structural layers 1, 3, 5 and 11 are separated by compound layers 9, 12 comprising at least one nitride, oxide or carbide of boron, magnesium or titanium. The compound layer 12 is preferably thin layer. The first side of the structure can be covered by a graphite sheet 13 of appropriate thickness (Fig. 2 and 3). Graphite is a well- -kπowπ moderator substance.

As it is shown in Fig. 3 it is advantageous to pre¬ pare combined structures comprising pairs of titanium structural layers 1, the pairs being divided by an inter¬ mediate covering layer 17 made of at least one nitride, oxide or carbide of boron, magnesium or titanium. The com¬ pound layers 9 divide the structure into parts comprising the pairs of the titanium structural layers 1. Each pair of the titanium structural layers 1 delimitates a system ar¬ ranged in its middle part and including either two shield structural layers 3 made of boron and between the last the intermediate structural layer 5 of beryllium or the magne¬ sium intermediate structural layer 11 in a space sequence as required by the given conditions, e.g. alternatively.

A preferred embodiment of the structure of the inven¬ tion comprises advantageously at least thirty-two structural layers 1, 3, 5, 11. These thin layer have thicknesses in the range from 0.0001 to 0.01 mm, e.g. in the sequence shown in Fig. 2. They are separated by compound layers 9 and 12 hav¬ ing thicknesses at most 0.0001 mm. This structure is prefer¬ ably at most 0.3 mm thick.

Of course, the structure as proposed by the invention may include structural layers of thicknesses higher than 0.01 mm. In some applications the layers are preferably se- lected to have thickness exceeding 0.01 mm, advantagously of about 0.1 mm. The choice of the dimensions depends on the task of realizing the structure, on the field of the appli¬ cation. The basis of the structure proposed by the invention is the titanium. This is a metal having seven isotopes whereamoπg five are stables. The average cross-section of the titanium (^Ti) is 5.8 barn for the thermal neutrons and this average value refers generally to the isotopes, too. The cross-section for reactor and fast neutrons is much smaller than the value mentioned. The titanium has the fol¬ lowing scheme of transformation (the percentage values given hereunder represent the natural isotopic composition of ti¬ tanium):

^Ti* (half-period 3.09 h) - ^i (7.99 %) -

- 22T1 (7.32 %) - ^i (^"73.98 %) - * Ti (5.46 %) -

- 2°Ti (5.25 %) - 22^Ti* (half-period 5.80 h)

(* - transformation with beta-decay and emission of gamma- -radiation.)

It can be seen, that the first neutron captured by the titanium causes a (n, gamma) reaction. In the first cap¬ ture reactions 94.75 % of the titanium atoms are not subject to the change of the atomic number. This means that under influence of the neutrons only about 5.25 % of the titanium will be converted into vanadium in the first stage of the process. It is very advantageous that in the second stage of this process about 89.29 % of the titanium applied at the beginning take part yet. The titanium isotope with mass num¬ ber 52 converts into vanadium with the half-period of this converting process less than 6 minutes. In the scheme men¬ tioned above the instable isotopes of the titanium (with mass numbers 45 and 51) are subject to beta-decay with gam- ma-emission. The vanadium shows the following scheme of transfor¬ mation (the percentage values given hereunder represent the natural isotopic composition of vanadium):

49,, ,__ ... ____. __, ,,„ _,_.._N 50

23V (half-period 330 days) - 23 (0.25 % , half- -period 5.10¹⁴ years) - ^^v ("-75 %) - 23

The last isotope shows half-period 3.75 min. and transforms by beta-decay with gamma-radiation. It can be seen that in this way the generation of the rather not desired isotope of vanadium with mass number 50 is practically avoided. The next stage of the transformation process provides chromium from vanadium, then the process results from chromium in mangaπum and the activation process ends on iron. The res- pective transformation data can be found in many different handbooks, so there is no need to recite them. Therefore the schemes of transformation are not shown further here. It is to noted, however, that the rather dangerous chromium iso¬ tope having mass number 51 ( _ΛCΓ) is present only in small amounts (the respective half-period makes out about 27.8 days) .

The transformation scheme can be continued by cobalt, nickel and copper having respective atomic numbers 27, 28 and 29. The probability of reaching the cobalt and further stages from titanium is very low. It should be mentioned the full scheme beginning from the titanium up to copper results always in metals of relatively high mechanical strength. The conclusion is that different products applied in radioactive environment, e.g. the containers, vessels, etc. can be made on the basis of pure titanium and applied long time in the conditions of the very intensive neutron and gamma-particle showers.

The elements of this transformation scheme have crys¬ talline lattice of cubic and orthorombic systems. As for the. titanium it should be mentioned that in a (n,2π) reaction it can be converted into scandium (_?.Sc).

This reaction has relatively low probability and therefore the transformation scheme in this direction leading to a

44 stable calcium isotope (_nnCa) is not dangerous.

5 According to the investigations the neutron capture processes result in the following change of the composition of a material initially composed of titanium:

1. After the capture of the first neutron: 94.75 % Ti and 5.25 % V.

10 2. After the capture of the second neutron: 89.29 %

Ti, 5.46 % V and 5.25 % Cr.

3. After the capture of the third neutron: 15.31 % Ti, 73.98 % V, 5.46 % Cr and 5.25 % Mπ.

4. After the capture of the fourth neutron: 7.99 % 15 Ti, 7.32 % V, 73.98 % Cr, 5.46 % Mπ and 5.25 % Fe.

5. After the capture of the fifth neutron: titanium vanishes, 7.99 % V, 7.32 % Cr, 73.98 % Mn, 5.46 % Fε and 5.25 % Co.

6. After the capture of the sixth neutron: titanium 20 and vanadium vanish, 7.99 % Cr, 7.32 % Mn, 73.98 % Fe,

5.46 % Co and 5.25 % Ni.

The process can be continued, however, the probabili¬ ty of lasting up to vanishing iron is rather low.

According to the invention between the titanium 25 structural layers 1 can form a sandwich structure together with a magnesium intermediate structural layer 11. The mag¬ nesium is advantageous because of forming aluminium by a transformation process:

,_n 24 - 25 - 26 - 27* 27 - 28*

30 ₁₂ ^M9 ~ 13^{A1 "}

28 - 29 - 30 - 31* , _ 31_p 14^bl 15^r

(* - transformation with beta-decay and emission of gamma- -radiation.) 5 The magnesium takes part with isotopes having mass numbers 24, 25, 26 and 27. The last isotope has half-period 9.5 min. , it transforms by beta-decay with gamma-radiation. The same transformation characterises the instable isotope of aluminium with mass number 28. It is obvious that alumi- 5 nium and magnesium can be used in structural applications requririπg high mechanical strength. The data evidence that a magnesium layer retains the mechanical strength during two neutron capture processes, and the resulting aluminium will survive a capture process more. The magnesium structure is,

10 however, advantageous because of low capture cross-section, i.e. the probability of a capture process is relatively low. Therefore magnesium with a neutron shield can long live in the environment of intensive neutron radiation.

The structure proposed by the invention includes the

15 titanium structural layers 1 as an outer protective cover. The cover receives the shower of the charged particles, too. As secondary structural substance magnesium can be applied. The outer surface of the titanium structural layer 1 limit¬ ing the structure from the first side is preferably covered

20 by titanium nitride.

The three-layer structure of the invention compris¬ ing two titanium structural layers 1 and a magnesium inter¬ mediate structural layer 11 therebetween is not applicable against neutron and gamma-radiation without further means

25 and shielding layers.

The neutron radiation generally comprises reactor and fast neutrons which require slowing down when securing ef¬ fective protection. This process is desired because of con¬ siderable increase in the cross-section for a neutron cap-

30 ture process. The problem is that the excellent neutron ab¬ sorbents as cadmium or gadolinium emit very intensive gam a- -radiation having energy range from 1 to 10 MeV. This gamma- -radiation emitted under influence of an intensive neutron shower can destroy any structural material except concrete

35 or lead used in thick shields. The last materials are prac- tially not applicable in the space technology. Therefore it is necessary to slow down the neutrons and this should be done by a process not based on the (π, gamma) reaction. It is also a novel feature of the invention that for slowing 5 down the neutrons the reaction (n, alpha) is preferably app¬ licable. This recognition results in the selection of the intermediate structural layer 5 to be covered by a shield structural layer 3 made of boron or lithium for slowing down the neutrons, transforming thereby the reactor and fast

10 neutrons into thermal neutrons absorbed by the intermediate structural layer 5. The features of the boron are more ad¬ vantageous than that of the lithium. The cross-section of the boron for the (π, alpha) reaction is 4000 barn. Under influence of the neutron shower the following process takes

15 place:

₁gB (n, alpha) ^Li

In this process energy Q = 2.8 MeV becomes free and will

20 result in heating up the material. A low intensity charac¬ teristic radiation is emitted also. The lithium is a stable element but the chemical features are not acceptable for a mechanical system. The boron acts therefore as a moderator emitting gamma-radiation which should be absorbed by a be-

25 ryllium layer forming the material of the intermediate structural layer 5. The alpha-particles emitted in the reac¬ tion recited above have energy about 1.5 MeV and they take up respective electrons from the atoms of the environment transforming thereby into helium. In a relatively low proba-

30 bility reaction the tritium (,H) can be deliberated, too. These processes mentioned form the disadvantage of the structure proposed by the invention.

The disadvantage is not so important. The advantage of the structure is that boron is applied which is a low

35 density material with relatively high strength and its melt- iπg point is also high (2030 °C). Therefore the armour plate comprising a boron layer is applicable in the space techno¬ logy.

The boron structural layer 3 secures thereby protec- tion against the neutron showers entering from the environ¬ ment. This protection is, however, not effective against the neutrons generated by the gamma-radiation impacting the boron layer and in the other layers . of the structure. Therefore is a rear boron layer necessary, as shown in the Fig. 1, 2 and 3.

The beryllium layer as intermediate structural layer 5 is also a very important feature of the present invention, because beryllium is an especially light metal having den¬ sity about 1.85 kg/dam . The advantage of applying beryl- lium is even the (gamma, n) reaction taking place therein according to the following scheme:

4 2

_□Be (gamma, n) ήe

The process results in deliberating energy Q = 1.65 MeV. The reaction takes place under the condition of impacting gamma- -radiation having energy at least 1.7 MeV. The neutrons have energy 110 keV. The last particles will be absorbed by the second shield structural layer 3 made of - as mentioned - boron.

The effective cross-section of beryllium for the process of capturing neutrons is low, about 0.01 barn but the scattering effect is relatively strong therefore beryl¬ lium is rather a neutron reflector than moderator. A thin layer of a heavy metal can improve the shielding effect of the structure built up as proposed.

The structure according to the invention comprises a titanium structural layer 1 on the first side where the ra¬ dioactive radiation strikes the body. It is preferred to prepare here an outer covering layer 7 of titanium-nitride. A thick layer made of e.g. rhodium can be also advantageous if applied together with the titanium-nitride layer or sepa¬ rately. The rhodium layer gives protection against strong optical radiation and it is capable of surviving one activa- tion process.

The titanium elements can show disadvantageous mecha¬ nical features in the high temperature range. The vanadium + + titanium alloy is free of this drawback. Therefore the alloy is also applicable, especially because of comprising the vanadium component forming the second stage of the process of activating titanium. The shield structural layer 3 should be made rather of pure boron associated, when ne¬ cessary with graphite. It is proposed to apply boron on gra¬ phite filaments - if the mechanical strength requirements allow to do so. The combined filament structure can be com¬ pacted by pressing the filaments together. This solution is advantageous because of ensuring place for helium deliberat¬ ing in the transformation process of the nuclei. Of course, boron can be sintered or compacted by other technologies, too. Another preferred possibility is to apply boron fila¬ ments covered with graphite and this structure can also be compacted by known methods.

In the intermediate structural layer 5 beryllium can ^■ be dispergated in magnesium. The first material is present in form of an oxide (BeO) and magnesium constitutes a plate. (Pure boron or boron in form of carbide or nitride can be dispergated also in the magnesium plate.) The shield struc¬ ture built up from the materials mentioned above should have thickness at least 0.3 mm. The thickness of the layers is not restricted, it depends on the circumstances of applying the protective shield structure proposed and can vary in the range from 0.0001 up to 0.1 mm.

The basic element of the structure proposed by the present invention can be recapitulated as follows: 1. layer: ^ comprising, when necessary _j-Λf and covered from the outer side by TiN and/or _ΛcRh;

2. layer: ,-B in pure form or dispergated in magnesium plate or in form of a cover prepared on graphite fila¬ ments or of a core of filaments covered with gra¬ phite;

3. layer: .Be in pure form or dispergated in a magnesium or copper plate;

4. layer: similar to the 2. layer;

10 5. layer: «_?Ti comprising, when necessary ~-,\l.

The layers described above are generally separeted by compound layers, made of oxide, nitride or carbide of boron, magnesium and/or titanium. Of course, a mixed compound layer may be also applied. The layers can be united by mechanical

15 means, if required or by appropriate adhesive substance. The basic structure as shown above is a low density body. The mechanical strength is high, the radiation up to very high energy range can not cause heavy damage to it. The layers slow down and absorbe practically all kinds of neutral and

20 charged particles. The activation process lasts short time, generally some minutes. The isotopes characterised by longer half-period are present only in very small, trace amounts. The protection against gamma-radiation is very effective for the energy range exceeding 1.7 MeV. If the effectiveness for

25 the lower energy range should be improved a thin layer of a heavy metal would be required, however, the structure itself secures protection also in this energy range.

The construction as proposed can be strengthened by the other sandwich structure described. The last comprises a

30 magnesium layer connected with respective titanium layers on each side. The magnesium layer can be applied together with and behind the five-layer structure shown above as the sixth layer followed by a further titanium layer, if necessary. The magnesium layer is prepared generally for improving the

35 mechanical features under the conditions when intensive sho- wers of neutral and charged elementary particles, nuclear fragments attack the shield structure proposed by the inven¬ tion.

The sandwich structures specified above can be appli- ed in a body comprising a plurality of these structures. The minimal thickness of the body should generally be 0.3 mm. It is advantageous to prepare at least one of the structures with structural layers in form of thin layers of thickness in the range from about 0.0001 mm to 0.01 mm, preferably about 0.001 mm. The thin layer structure is advantageous be¬ cause of the backscatter phenomenon caused by the gamma-ra¬ diation on impacting. As example, construction units with the following sequence of the structural layers 1, 3, 5, 11 can be prepared (the chemical elements are here shown with- out respective atomic numbers):

Ti - B - Be - B - Ti -; Mg -; Ti - B - Be - B - Ti Ti - B - Be - B - Ti -; Mg -; Ti Ti - B - Be - B - Ti Ti - B - Be - B - Ti -; Mg -; Ti - B - Be - B - Ti -; Mg Ti - B - Be - B - Ti; followed by any one of the above structures The titanium structural layers 1 can form together with the magnesium intermediate layers 11 a structure where¬ in at least one pair of layers Ti + Mg is present and the structure is closed by a TiN layer. Magnesium can be mixed (alloyd) with copper, the other additives are rather to be avoided.

The structure of the invention can be completed with further protecting means. As a very effective protecting element the application of a graphite plate of thickness ex¬ ceeding 0.5 mm is especially advantageous.

As mentioned above, it is especially preferred to build up the structure of the invention with at least thir¬ ty-two structural layers wherein at least one of the basic construction units defined above is made of thin titanium, boroπ and beryllium layers having thickness in the range from 0.0001 to 0.01 mm, preferably about 0.001 mm. The gene¬ ral thickness of this system should be however higher than 0.3 mm (of course, a thicker construction can be built up, wherein more layers have thickness exceeding 0.1 mm and the summarized thickness is at least 0.5 mm).

The structures proposed by the present invention can be produced without technological difficulties. They are not too expensive, they ensure high level safety for long terms. The density is relatively low, which is of outraging impor¬ tance in the space technology. The structure shows high re¬ sistance against chemical substance and it can be applied in preparing containers receiving nuclear waste, shields to be used on aeroplanes or in space technology means for ensur- ing protection against different kinds of radioactive radia¬ tion.

Claims

CLAIMS :

1. A structure for shielding radioactive radiation, said structure having first and second sides, said radioac¬ tive radiation striking said structure on said first side, said structure comprising at least three structural layers, wherein each of the first two structural layers, taken in sequence from said first side toward said second side, comp¬ rises an element which converts at least a part of a first kind of radioactive radiation into a second kind thereof, characterized in comprising at least two titanium structural layers and a middle part arranged between said titanium structural layers, wherein one of said titanium structural layers constitutes said first side and said middle part includes at least one intermediate struc¬ tural layer made of a metal selected from the group consist¬ ed of beryllium and magnesium.

2. The structure as set forth in claim 1, characterized in that said intermediate structural layer comprises beryllium and is separated from said titanium structural layers by shield structural layers prepared with boron for slowing down neut- rons.

3. The structure as set forth in claim 2, characterized in that said intermediate structural layer is made of beryllium dis¬ pergated in form of beryllium oxide in a metal selected from the group consisted of magnesium and copper.

4. The structure as set forth in claim 2, characterized in that said shield structural layers consist of boron dispergated in magnesium.

5. The structure as set forth in claim 2, characterized in that said shield structural layers consist of filaments made of a first material selected from the group consisted of boron and graphite and covered with a layer made of the second material selected from said group.

6. The structure as set forth in any of claims 1 to

5, characterized in that said structural layers are separated by respective compound layers prepared with a compound selected from the group coπ- sisted of oxides, nitrides and carbides of at least one ma¬ terial selected from the group consisted of boron and tita¬ nium.

7. The structure as set forth in any of claims 1 to

6, characterized in comprising an outer covering layer consisted of at least one material selected from the group consisted of titanium-nit¬ ride and rhodium, said outer covering layer being prepared on said first side.

8. The structure as set forth In any of claims 1 to 7, characterized in comprising at least thirty-two structural layers forming a body of thickness at least about 0.3 mm, wherein said struc¬ tural layers are separated by respective compound layers prepared with at least one compound selected from the group consisted of oxides, nitrides and carbides of at least one material selected from the group consisted of boron and ti¬ tanium, said compound layers having thickness in the range up to about 0.001 mm.

9. The structure as set forth in any of claims 1 to 8, characterized in comprising at least thirty-two structural layers having thicknesses at most 0.01 mm.

10. The structure as set forth in any of claims 1 to 9, characterized in comprising a graphite sheet covering said first side.