WO2012013372A1

WO2012013372A1 - Pulsed spallation neutron source

Info

Publication number: WO2012013372A1
Application number: PCT/EP2011/056959
Authority: WO
Inventors: Oliver Heid
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-07-26
Filing date: 2011-05-02
Publication date: 2012-02-02
Also published as: DE102010032216A1; DE102010032216B4

Abstract

The invention relates to an apparatus for generating a temporally smoothed neutron flux in particular for a nuclear reactor operated in a subcritical mode. The apparatus comprises a particle accelerator, which generates a pulsed particle beam with a very high pulse repetition rate of for example 3 kHz. The particle beam impacts a spallation target, where it causes a neutron flux which is likewise pulsed to be ejected, which for its part enters a neutron moderator. The neutron moderator has a known distribution of residence times. Temporal smoothing of the neutron flux leaving the moderator is achieved by the residence time and the pulse repetition rate being matched to each other either by selecting a moderator that has a suitable distribution of residence times or by setting an appropriate pulse repetition rate f_pulse for a specific, solid moderator. By way of example, 1/f_pulse=T_v,ocan apply, where T_v,o denotes the most likely residence time of the distribution.

Description

description

Pulsed spallation neutron source

The invention relates to a neutron source for use in a subcritical nuclear reactor.

In a subcritical nuclear reactor, the nuclear reactions take place without the reactor going into the critical state. Instead of maintaining a self-contained chain reaction, an external neutron source is used to provide the neutrons needed for the nuclear reactions. This purpose is fulfilled, for example, by a particle accelerator whose particle beam is directed towards a spallation target, from which neutrons are subsequently ejected, which finally reach the subcritical nuclear reactor in order to effect the nuclear reactions there. This technology is known per se. Such subcritical nuclear reactors that utilize a particle accelerator are referred to as ADS (Accelerator Driven System) reactors, or ADSRs for short.

Subcritical reactors have a number of advantages. For example. they can be used to destroy heavy isotopes contained in the spent fuel of conventional nuclear reactors, while at the same time generating energy. Theoretically, the transuranium contained in nuclear waste can be split, which in turn energy can be obtained, while the resulting cleavage products are less durable than the transuranic. Further development of the subcritical reactors and their driving accelerator driven neutron sources is therefore of great interest.

For the appropriate accelerator structures, a distinction must be made between normal-conducting and superconducting accelerators: The normal-conducting accelerators must be operated pulsed in practice, since the required sizes lead to electric fields in the accelerator cells (resonators or cavities) which cause an excessive power loss in the surrounding field-enclosing conductor structures, for example in the resonator walls , However, a pulsed accelerator operation and, as a consequence, a pulsed neutron flux from the spallation target has the result that the reactor core fluctuates greatly in energy production and is thus heavily loaded thermally and mechanically.

This problem could be solved using a superconducting accelerator. Although this could be operated continuously and thus avoid the mentioned problem of the normal-conducting accelerator, a compact size would not be feasible. For example. for the European spallation neutron source ESS would result in a length of 630m. Due to this disadvantage, the use of a superconducting accelerator for commercial use is almost impossible.

It is therefore an object of the present invention to provide a compact device for generating a time-smoothed neutron flux whose use, for example. With a subcritical nuclear reactor does not have the disadvantages mentioned.

This object is achieved by the inventions specified in the independent claims. Advantageous embodiments emerge from the dependent claims.

The solution according to the invention is based on a normal-conducting accelerator operated in pulsed fashion with the aid of a correspondingly established control device, whose pulsed particle beam is directed onto a spallation target from which neutrons are knocked out in a known manner. In this case, an extremely high pulse repetition rate is selected, for example. in the order of> 3kHz. Downstream of the spallation target, ie between taget and reactor core, a neutron moderator with a known neutron residence time is arranged, with which it is to be achieved that the neutron flux from the target reaching the subcritical reactor core is time-smoothed, that is, from the moderator into the target Reactor core neutron flux does not vary significantly. For example. would be an order of magnitude of up to about 20% acceptable. In principle, however, this of course depends on the respective intended application.

This is achieved by either using a neutron moderator whose neutron residence time, that is to say a size dependent on the material and dimensions of the moderator, approximately corresponds to the selected pulse repetition rate, or the pulse repetition rate, for example, with the aid of a corresponding control device for the particle accelerator the known Neutronenverweilzeit is adjusted. Furthermore, the residence time is, of course, also dependent on the speed of the neutrons and thus to a certain extent on the target. An exact residence time curve is, for example.

obtained by simulation of the overall system.

For example. a pulse repetition rate or, accordingly, a time interval between two successive pulses of the pulse sequence is selected, which corresponds approximately to the neutron dwell time. That at a repetition rate of, for example, 3 kHz, the dwell time in the moderator should be approximately 1 / (3 kHz) «0.33 ms or more. As a result, the neutron flux from the moderator, despite the pulsed particle beam and despite the pulsed neutron flux leaving the target, does not vary significantly over time and, accordingly, the reactor core driven by the neutron flux does not perform excessive power fluctuations.

Due to a possibly more complex geometry of the moderator and / or the entire arrangement, the running ege of the individual neutrons by the moderator more or less differ so that it is no longer possible to speak of a single neutron residence time but of a distribution of the neutron residence times between a minimum value and a maximum value. In this case, there are several possibilities for adjusting the dwell time and the repetition rate, as explained in connection with the figures.

The device according to the invention for generating a neutron flux N _mo d has:

a particle accelerator having a control device, the control device being adjustable such that with the particle accelerator a pulsed particle beam S comprising a pulse train having a multiplicity of pulses with a pulse repetition rate f _pu _{s s can be} generated,

a spallation target which can be positioned in the particle beam S and from which a first neutron flux N _tar comprising a multiplicity of neutrons can be _knocked out by the particle beam S,

a neutron moderator having a known distribution

X _n (T _v ) of Neutronenverweilzeiten T _v , wherein the target and the moderator are arranged to each other such that at least a portion of the first neutron flux N _ta r enter the moderator and can pass through it, so that the neutron flux to be generated N _mod leaves the moderator.

The pulse repetition rate f _pu is and the distribution X _n (T _v ) of the neutron residence times T _{v of} the moderator are now matched to one another such that the neutron flux N _raod leaving the moderator is temporally smoothed.

For this purpose, the control device can be set such that the pulse repetition rate f _pu i _s corresponds to the neutron residence time T _v .

Specifically, the control device is adjustable so that for the time interval

between two successive pulses of the particle beam S, the following applies: AT _puls = 1 / f _pul3 > T _Vjrain , where T _v , "" is the minimum residence time of the distribution X _n (T _v ) of the neutron residence times T _v .

It is valid for the time interval

between two successive pulses of the particle beam S: AT _pulse = 1 / f _pulse <T _Vitnax , where T _v , _m a _X the maximum

Residence time of the distribution X _n (T _v ) of the neutron residence times T _v .

Due to this relation between AT _pulse, _v, _m and T _v, _ma x si ^¬ cher is found that the triggered by a first pulse of the particle beam neutron flux is N does not yet subsided _m0d if another neutron flux by the subsequent pulse of the particle beam N _{mod is} generated.

The control device may also be adjustable such that for the time interval

between two successive pulses of the particle beam S, the following applies:

A puis ⁼ / puis ~ ^T v, o 'where T _v, o the most likely ^¬ dwell time distribution of the X _n (T _v) of the Neutronenverweilzeiten T _v.

A pulse repetition rate f _pu is selected of at least 3 kHz.

The moderator is arranged in such a way that the neutron flux N _mod leaving the moderator passes into a reactor core of a nuclear reactor, which operates in a subcritical manner in particular.

Due to the smoothing of the neutron flux, the nuclear reactor or its core is neither thermally / mechanically heavily loaded despite the use of a pulsed particle beam nor fluctuates the energy production beyond a tolerable level.

The particle beam is advantageously a proton beam.

In the method according to the invention for generating a neutron flux N _mod , a particle accelerator generates a pulsed particle beam S comprises a pulse train having a plurality of pulses with a pulse repetition rate f _u is · The pulsed particle beam is directed to a spallation target, wherein a plurality of neutron extensive first neutron flux is knocked tar from the target by the particle beam. At least part of the first neutron flux N _ta r enters a neutron moderator and traverses it so that the neutron flux N _m or the moderator leaves to be generated. The moderator has a known distribution X _n (T _v ) of neutron residence times T _v . The neutron flux N _mo d leaving the moderator is now temporally smoothed by matching the pulse repetition rate f _pu _s and the distribution X _n (T _v ) of the neutron residence times T _{v of} the moderator.

The adaptation is carried out in such a way that for the temporal

distance

between two successive pulses of the particle beam (S): A T _puls = 1 / f _pulse > T _Vrrain , where T _v , min is the minimum residence time of the distribution (X _n (T _v )) of the neutron residence times (T _v ).

Furthermore applies to the time interval

Zvi ^¬ rule two successive pulses of the particle beam (S): AT _PUL3 = 1 / f _pulse <T _{V max,} where T _v, _m ax the maximum stay time of the distribution (X _n (T _v)) of the Neutronenverweilzeiten (Tv) is ,

In particular, the adaptation is carried out such that for the time interval

between two successive pulses of the particle beam (S):

- 1 / f _pu i _s ^{Ä T} v, o 'where T _v, o the most likely ^¬ dwell time of the distribution is (X _n (T _v)) of the Neutronenverweilzeiten (T _v).

In summary, the object is achieved by the use of a particle accelerator which is operated at extremely high pulse repetition rate, in combination with a suitable neutron moderator, which leaves the target. smooths the neutron flux over time. The neutron moderator or the neutron residence time in the moderator and the pulse repetition rate are adapted to each other. In order to achieve the high pulse repetition rate, an RF transmission arrangement is required, the correspondingly low filling times of the HF-resonators of the accelerator allows route (maxi ^¬ male RF power at varying load). A solution to this problem is not the subject of the present invention, but can be found in DE 10 2009 053 624. There, RF cavities or resonators as well as an accelerator equipped with these HF cavities are indicated, wherein in particular the problem of the coupling of HF Performance in the RF cavity is in the foreground. It is therefore with regard to the problem of low filling times of the resonators, which is solvable with a suitable device for coupling the RF power in the resonators, on the

DE 10 2009 053 624. Further advantages, features and details of the invention will become apparent from the embodiment described below and with reference to the drawings.

Showing:

1 shows the inventive device for generating a temporally smoothed neutron flux for a subcritical nuclear reactor, Figure 2 shows the temporal behavior of the pulsed particle beam and the various neutron fluxes and

FIG. 3 shows an exemplary distribution of neutron dwell times.

FIG. 1 shows a device for generating a time-smoothed neutron flux N. Shown are the core 110 of a subcritical nuclear reactor 100 and a Spallation target 120 and a neutron moderator 130. The target 120 is irradiated with a proton beam S from a particle accelerator 200, wherein the particle accelerator 200 comprises an ion source 220, an accelerator path 230 with at least two RF resonators 231, 232, and a control device 210 for controlling the RF Resonators 231, 232 has.

The RF resonators 231, 232 of the accelerator section 230 are arranged one behind the other in the beam direction and cause the acceleration of the particles removable from the ion source 220, for example hydrogen nuclei or protons.

By means of the control device 210, RF fields are generated in the RF resonators 213, 232, which are used to accelerate the

Ion source taken particles serve. This basic mode of operation of such accelerators, including the manner of generating the RF fields in the RF resonators, for example using a klystron, is known and will not be described further here. The wording that

"The control device 210 generates the RF fields" should include this per se known mode of action, ie, the control device 210 includes both the required for generating the RF fields in the resonators components including borrowed eg. The klystron and possibly used waveguide etc., as well as an electronic assembly, which performs the various necessary calculations and the actual control of the components. With the aid of the control device 210, the RF resonators 231, 232 as described in DE 10 2009 053 624 can be controlled separately, ie the RF field of the first RF resonator 231 can be generated independently of the RF field of the second resonator 232. Due to the thus achievable RF decoupling, it is possible to control the individual RF cavities 231, 232 independently of one another with the aid of the control device 210, as a result of which the accelerator 200 is operated more flexibly and flexibly to the respective desired acceleration to be achieved. can be adjusted. The adaptation is more flexible than with an accelerator, in which the RF cavities in the RF region are coupled together, so that the control of one RF cavity simultaneously influences the RF fields in the adjacent RF cavity. Nevertheless, the invention set forth here can also be implemented with an accelerator in which the individual RF resonators 231, 232 are not driven separately. Regardless of whether the HF-resonators 231, separately controlled 232, the control device 210 is such inserted ^¬ oriented such that the particle 200 is operated in pulsed mode, ie that of the particle accelerator 200 removable particle beam S consisting of follow one another constricting as a pulse train bunches or pulses, so-called bunches.

In FIG. 2A, a pulse train P (t) of the particle beam or the particle flux density of the particle beam is shown purely by way of example and in arbitrary units as a function of time t.

As described in the introduction, the particle beam S in the spallation target 120 of the nuclear reactor 100 causes neutrons to be ejected from the target 120. Due to the pulsed particle beam S, the neutron flux N _tar leaving the target 120 is also pulsed (FIG. 2B), ie the neutron flux N _tar has a similar temporal behavior to that of the pulsed particle beam S. This pulsed neutron flux N _tar now first enters into a neutron moderator 130 in which the neutrons are decelerated. The neutron flux ^ c _d (FIG. 2C) leaving the moderator 130 reaches the core 110 of the nuclear reactor 100, where finally the desired nuclear reactions take place. N _CTO d indicates the number of neutrons leaving the moderator 130 per unit of time. The temporal behavior of N _raod is dependent on the distribution of the residence times T _v in the moderator 130. The residence time describes the period of time which require the neutrons to pass through moderator 130. The residence time is thus dependent on the initial speed of the neutrons leaving the target 120 or entering the moderator 130, the braking capacity of the moderator 130 and the length of the path the neutrons travel through the moderator 130, ie its geometry and dimensions.

3 shows by way of example and in a greatly simplified manner the distribution of the residence times T _{v of} the neutrons knocked out of the target 120 by a pulse of the particle beam S in the moderator 130. The number of neutrons X _n is plotted over the residence time T _v . The curve is therefore to be interpreted such that, for example, a certain number X _n , _x remains in the moderator for a period T _v , i before it enters the reactor core. Accordingly, there remains a number X _n , 2 for a period T _v , 2 in the moderator, etc. The area under the curve thus ultimately corresponds to the total number of neutrons knocked out of the target by a pulse of the particle beam (provided that none of these neutrons are lost goes). In FIG. 3, it has been assumed that the trajectory X _n (T _v ) exhibits a quadratic behavior or is parabolic, ie in principle

X _n (T _v ) = - Cl · (T _v - T _v0 ) ² + 02, where Cl, C2 and T _v , o are constants that depend on the choice of moderator.

Of course, in practice, the shape of the curve essentially depends on the geometry and material of the moderator 130, the geometry and material of the target 120, as well as on the time length and shape of the pulses of the particle beam S. T _v , _m in or Tv, max denote the minimum and the maximum residence time of the neutrons in the moderator. T _v , o generally designates the residence time which the predominant number of neutrons spends in the moderator or the residence time for which the curve Xn has a maximum. T _v , o can therefore be called the most probable residence time. The neutron flux N _mo d (t) caused by a single pulse has qualitatively substantially similar temporal behavior as the curve shown in FIG. Assuming that the pulse hits the target 120 at time t = 0, the first neutrons leave the moderator 130 at a time t _v , min. At a time t _v , o, the neutron flux exiting the moderator is maximal, while the neutron flux at time t _v , _ra ax has subsided again. By means of a suitable choice of the moderator 130, in particular with regard to material, geometry and arrangement relative to the target 120, the profile curve according to FIG. 3 or the distribution of the residence times T _v can be influenced. Conversely, a certain host 130 is characterized by a particular _v T _v neu- ronenverweilzeit or by a certain distribution of residence times T. In an extreme case, an arrangement would be conceivable with which the distribution of the residence times is not comparatively wide, as in FIG. 3, but is essentially reduced to a comparatively narrow individual peak at T _v = T _v , o. For example. could the presenter 130 in

View beam direction seen behind the target 120 may be arranged so that only meet the target in the beam direction straight leaving neutrons on the moderator, so that these neutrons essentially the same time period T _v , o need to traverse the moderator.

For the course of the N _mod curve in FIG. 2C, it has now been assumed that the distribution of the residence times T _v or, associated therewith, the temporal behavior of the neutron flux N _m0 d triggered by a pulse corresponds to the curve of FIG.

FIG. 2C shows, based on the behavior according to FIG. 3, the neutron flux rao _d resulting from a multiplicity of pulses. It is indicated by the dashed curves for each pulse of separately triggered by this pulse neutron flux. The entire neutron flux N _m od results here by a simple summation of the individual neutron fluxes.

The total, superimposed neutron flux N _m0 d from the moderator 130 according to FIG. 2C has a completely different time behavior than the neutron flux triggered by a single pulse according to FIG. 3: after a certain number of pulses or after the transient phase, the neutron flux is N. _m0 d temporally smoothed and varies between a minimum value N _mod , _min and a maximum value N _mod , _raax with AN _mod = N _{mod; iIlax} - N _{mod | [tlln} .

The difference between the minimum and the maximum value of the neutron flux _m0 dA ie the fluctuation AN _m0 d of the neutron flux now depends on several factors. The interaction between the time interval AT _pu _{s of} the pulses of the particle beam on one side and the distribution of the residence times T _v for the neutrons of a pulse according to FIG. 3 on the other side ultimately determines the temporal behavior of the neutron flux N _m0 d-

Thus, in simple terms, if the maximum residence time T, m _a x is less than the time interval AT _pu _{s of} the pulses of the particle beam, the minimum value N _mod , _m "of the neutron flux N _{raod becomes} comparatively small, ie N _mod , _m in ~ 0, since the neutron flux triggered by a first pulse has already decayed when the neutron flux triggered by the following pulse occurs. So there is no accumulation of neutron fluxes and no settling. Accordingly, the fluctuation ΔN _mod of the neutron flux N _mod leaving the moderator 130 is very large, which should be avoided as explained in the introduction. Conversely, the behavior shown in the Figure 2C can be achieved if we have _v, max A _p u _s. In the _typical example there, the ratio Ty, ma · AT _pU i _{s is} approximately 3: 1.

According to the invention, the moderator and the pulse repetition rate are now matched to one another. In a fixed moderator with known properties in particular with respect to the Distribution X _{n of} the neutron residence times T _v can be adjusted by means of the control device 210, the pulse repetition rate f _pu i _s accordingly. Conversely, the selection and interpretation of the presenter 130 may also be at a desired pulse repetition rate

of the particle beam S, ie the type or the material as well as the dimensions and geometry of the moderator 130 are selected so that the distribution X _{n of} the residence times T _{v of} the neutrons of the neutron flux N _tar from the target 120 in the moderator 130 to the Pulse repetition rate is adjusted.

In both said cases of adaptation, it could be used as an adaptation criterion such that the minimum residence time T _v , min of the distribution X _n (T _v ) corresponds at least to the time interval ATp _U i _s between two successive pulses of the particle beam, ie AT _pulse = 1 / f _pulse > T _v _ _min . A further limiting criterion, which is also used to create the

Diagram 2C was used, for example.

Tv, _min * ^AT P _U I _S ^ ^T v, _ma x or concrete AT _pulse * T _v _ ₀ . Depending on the width of the distribution of residence times in this case, ÄT _pu i _s must correspond more or less exactly T _v , o. The control device 220 is now set up so that it has an extremely high pulse repetition rate of eg.

fpuis> 3kHz generated.

The high pulse repetition frequency of> 3kHz naturally requires the shortest possible filling times of the HF energy in the HF

Cavities of the accelerator, ie a maximum transmission power. This is a worthwhile goal, especially with high-performance accelerators. The power absorbed by the particle beam in the cavity should, as is known, also dominate the power loss in the accelerator in the interest of high efficiency. Thus, during the filling or transient phase, ie without particle beam, the entire available transmission power can be used to fill the cavity, so that short filling times are possible. The RF concept of the accelerator must be designed so that as much RF power as possible is available during the filling phase, ie with variable HF load. A device suitable for this purpose, including the suitable HF cavity, is described, for example, in DE 10 2009 053 624.

Claims

claims

1. A device for producing a neutron flux (N _moc i) comprising:

- a particle accelerator (200) with a control device (210), wherein the control device (210) is a ^¬ adjustable such that the particle (200) is a pulsed particle beam (S) comprising a pulse train having a plurality of pulses with a pulse repetition rate fpuis is producible,

- a spallation target (120), the (S) is po ^¬ sitionierbar in the particle beam and from which, by the particle beam (S) a plurality of neutron extensive first neutron nenfluss (N _AR) is herausschlagbar,

- a neutron moderator (130) having a known distri ^¬ lung (X _n (T _v)) of Neutronenverweilzeiten (T _v), where ^¬ at the target (120) and the moderator (130) such zuein ^¬ other are arranged, that at least part of the first neutron flux (N _ar ) can reach the moderator (130) and pass through it, so that the neutron flux (N _moc i) to be generated leaves the moderator,

characterized in that the pulse repetition rate (f _pu i _s ) and the distribution (X _n (T _v )) of the neutron residence times (T _v ) of the moderator (130) are coordinated such that the neutron flux leaving the moderator (130) (N _moc i) is temporally smoothed.

2. Apparatus according to claim 1, characterized in that the control device (210) is adjustable such that the pulse repetition rate (f _pu i _s ) of the Neutronenverweilzeit (T _v ) ent ^¬ speaks.

3. A device according to claim 1, characterized in that the control device (210) is adjustable such that for the time interval

) between two successive pulses of the particle beam (S):

ÄT _pulse = 1 / f _pulse> T _{V; min,} where T _v, min is the minimum residence time of the distribution (X _n (T _v)) of the Neutronenverweilzeiten (T _v).

4. Apparatus according to claim 3, characterized in that the control device (210) is adjustable such that for the time interval

) between two successive pulses of the particle beam (S): ΔT _pulse = 1 / f _pulse <T _{V; max} , where T _v , max is the maximum residence time of the distribution (X _n (T _v )) of the neutron residence times (T _v ) is.

5. Device according to one of the preceding claims, character- ized in that the control device (210) is adjustable such that for the time interval

f _pU i _s ) between two successive pulses of the particle beam (S): AT _puls = 1 / f _pulse «T _{V; 0} , where T _v , o is the most probable residence time of the distribution (X _n (T _v )) of the neutron residence times ( T _v ) is.

6. Device according to one of the preceding claims, characterized in that the pulse repetition rate is (f _pu i _s) to ^¬ least 3kHz.

7. Device according to one of the preceding claims, characterized in that a nuclear reactor (100), ^{insbesonde ¬} re a subcritical nuclear reactor (100) is provided with a reactor core (110), wherein the moderator (130) is arranged such that the the neutron flux (N _moc i) leaving the moderator (130) enters the reactor core (110).

8. Device according to one of the preceding claims, characterized in that the particle beam is a proton beam.

9. Device according to one of the preceding claims, characterized in that the particle accelerator (200) at least two in the beam direction successively arranged RF resonators (231, 232), through which the pulse train comprising a plurality of particle pulses can be accelerated.

10. A method for generating a neutron flux (N _{mo c} i) r in the

- a particle accelerator (200) comprises a pulsed particle beam (S) is produced comprising a pulse train having a plurality of pulses with a pulse repetition rate f _pu i _s,

- The pulsed particle beam (S) is directed to a Spallationstarget (120), wherein from the target (120) by the particle beam, a plurality of neutrons comprehensive first neutron flux (N _ar ) is knocked out,

- (N _ar) reaches at least a portion of the first neutron flux in egg ^¬ NEN neutron moderator (130) and these traverses, so that the to-generating neutron flux (N _{mo c} i) leaves the Modera ^¬ gate, wherein the moderator (120) a known Ver ^¬ division (X _n (T _v)) comprises of Neutronenverweilzeiten (T _v), characterized in that the moderator (130) leaving ^¬ send neutron flux (N _{mo c} i) is smoothed in time by the pulse repetition rate (f _pu i _s ) and the distribution (X _n (T _v )) of the neutron residence times (T _v ) of the moderator (130) are coordinated.

11. The method according to claim 10, characterized in that the adaptation is carried out such that for the temporal

distance

) between two successive pulses of the particle beam (S): AT _pulse = 1 / f _pulse > T _{V; min} , where T _v , _{m is} the minimum residence time of the distribution (X _n (T _v )) of the neutron residence times (T _v ) is.

12. The method according to claim 11, characterized in that the adaptation is carried out such that for the time interval

) between two successive ones

Pulses of the particle beam (S) are: AT _puls = 1 / f _pulse <T _{V; inax} , where T _v , max is the maximum residence time of the distribution (X _n (T _v )) of the neutron residence times (T _v ).

13. The method according to any one of claims 10 to 12, characterized in that the adaptation is carried out such that for the time interval

) between two successive pulses of the particle beam (S): A ^T puis = 1 / ^T fpuis ~ v 'o' where T _v, o the most likely ^¬ dwell time of the distribution is (X _n (T _v)) of the Neutronenverweilzeiten (T _v).