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Surrogate experiments

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The ACEN group is at the origin of a complete renewal of an original technique for the determination of neutron induced reaction cross-sections for short lived target nuclides.

This technique consists the use of an alternative reaction to produce the same compound nucleus as in the direct neutron induced reaction and to measure the probability of different decay modes: fission or gamma cascade emission. This is particulary pertinent in the case of short lived target nuclides.

A typical actinide target for direct neutron induced fission studies is of 0.2 mg of active material, i.e. about 5 1017 target atoms. This represent an alpha activity of 40 kBq for a 233U target (T1/2: 0.25 million years), 1.5 MBq for a 243Am target (T1/2: 7370 years), and grows to 400 to 600 MBq for a 243Cm or a 244Cm target (T1/2: 29 and 18 years). More, the target activity would be up to 150 GBq for a 233Pa target (T1/2: 27 days) !

One can produced the same excited compound nucleus as with a 243,244Cm target using a transfer reaction from a 3He beam on a hundred times less active 243Am target. In the case of 233Pa, one can use a 232Th target, with a total activity less than one Bq ! the problem of direct neutron measurement of short lived nuclei.

The principe of the surrogate method is described here. It had been used to determine neutron induced fission and radiative capture cross sections.

We first used this method to determine the fast neutron induced fission cross section of 233Pa, which is a key nucleus for the generation of the fissile 233U uranium isotope in the thorium based fuel cycle.

Then we studied the possible extension of the surrogate method to the radiative capture cross section of 233Pa.

Later on, we applied the surrogate method to the determination of fission cross section of minor actinides.

We investigated the relevance of the extension of the surrogate method for the determination of neutron radiative capture cross section. For theses studies we used rare earth material targets for which some direct neutron cross sections were known with a high precision.

All this experimental program has been performed at the ALTO-Tandem facility of the IPN Orsay.

More recently these investigations have been pursuited using (d,p) transfer reaction on a 238U target at the cyclotron of the Oslo university.

Our surrogate programme is continuing with a planned experiment in April 2017 at the ALTO-Tandem facility of the IPN Orsay, using a 240Pu target.

Related PhD works:

M. Petit : Mesure de la section efficace de fission du 233Pa, Bordeaux, Oct. 2002
S. Boyer : Dans le cadre du nouveau cycle de combustible 232Th/233U, détermination de la section efficace de capture radiative 233Pa(n,γ) pour des énergies de neutrons comprises entre 0 et 1 MeV, Bordeaux, Oct. 2004
G. Kessedjian : Mesures de sections efficaces d’actinides mineurs d’intérêt pour la transmutation, Bordeaux, Nov. 2008
G. Boutoux : Sections efficaces neutroniques via la méthode de substitution, Bordeaux, Nov. 2011
Q. Ducasse : Étude de la méthode de substitution à partir de la mesure simultanée des probabilités de fission et d’émission gamma des actinides 236U, 238U, 237Np et 238Np, Bordeaux, Oct. 2015