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Reference measurements for neutron data standards


Specific Objectives

- To perform reference measurements on new and established standards and to improve measurements where large discrepancies exist in present data libraries;

- To perform reference measurements which provide the basic input data to model calculations used in the evaluation of neutron standards data.
Planned Deliverables

- Complete the first measurement campaign on the 10B-branching ratios in the
1keV to 1MeV neutron energy range. Send data to evaluators and NEA data bank.
Publication in refereed journal.

- Complete the analysis of the 10B(n,tot) cross section measurements at the IRMM VdG and publish the results. Data will be sent to the databank.

- Complete the 239Pu(n,f) analysis for single resonance's and send data to NEA databank and evaluators.

- Complete the neutron multiplicity and spectrum evaluation for 235U and 252Cf.
Publication of the results obtained within the ISTC collaboration on neutron multiplicity measurements.

- Publication of the results obtained for 238U and 237Np.

- Continue the measurements on 251Cf(n,f) at the high flux reactor of ILL.

Summary of deliverables made by: 31/12/2001

Neutron data standards help ensure correct measurements for reactor safety, waste minimisation, and transmutation and environmental protection. The major customers for this project are the OECD and Nuclear Industry.

- 10B(n, a) branching ratio: A first experiment was performed at GELINA from March to July 2001 whereby the gamma flash and an (epi-)thermal background were a severe problem and required additional shielding. The experimental set-up was afterwards moved to a 60 m flight path station to reduce the gamma flash and boron shielding was added. The preliminary results of the first measurement, if the full 4-pie solid angle was taken, showed good agreement with the ENDF/B-VI evaluation. Van de Graaff measurements at higher n-energies resulted in preliminary data for several neutron energy points up to 3.8MeV both for the ratio as well as for the separate cross sections. The latter showed significant disagreement with the ENDF/B-VI database. The branching ratio results were presented (upon invitation) at NIST, USA. Due to the very large anisotropy also the angular distribution at higher neutron energies were determined.

- 239Pu(n,f): Information exchange with and data were sent to CEA, France evaluators on previous 235U(n,f) results. So far this evaluation has given consistent agreement with prompt neutron measurements. The 239Pu(n,f) data for individual resonance's were re-analysed in order to finalise the results. 242Pu(SF) measurements were started to determine the tritium yield.

- Collaboration with ILL: A final report of the contract for the data analysis of the first 251Cf(n,f) experiment was written. Due to problems with the sample quality, the remaining 251Cf samples were reprocessed (completed in August 2001). The also acquired LCP-data were analysed and a publication of the results is being prepared.

- ISTC collaboration with Russian institutes: A prolongation of the project was accepted and data processing of a former experiment progressed further. The Russian authorities also approve the prolongation although with very long delay. The preparation of the additionally foreseen experiment already started.

- 252Cf(SF): For the moment no further activity is planned and the delivery of new Cf-targets is not foreseen before next year. Results of pulse height defect measurements for different gas mixtures using a Cf-target will be used for the 234U analysis.

- Neutron multiplicity and spectrum measurements: A new activity started with Romania on the modelling of fission neutron spectra based on IRMM fission yield measurements. Ingredients are our fission yield data to improve the calculation of the fission neutron spectra as a function of incident neutron energy. Calculations of the neutron multiplicity and spectra for 238U(n,f) and 237Np(n,f) were performed by the scientific visitors from Romania. First results are very promising and already yielded an improved model code.

- Improvement and development of new methods: First tests started on an axial ionisation chamber with splitter anode for charged particle identification. This device should be used to identify n-rich fission products and detection of their subsequent delayed n-emission. Also pulse height defect measurements have been performed for the CF4 gas mixture, which will be used for the analysis of the 234U(n,f) data.

- Cm(n,f): Measurements of the newly arrived isotope 245Cm(n,f) were performed at ILL, Grenoble. The data analysis was done and the results will be compared to 246Cm(SF). New measurements on 247Cm(n,f) were performed at ILL, the data were analysed and the ternary á-yield was determined. To enlarge the neutron energy range also measurements are foreseen at GELINA.

- Fluency inter-comparison (Collaborations: BIPM , CCRI and PTB ): Measurements at 1.2 5.0 and 14.8 MeV were performed at PTB and the data analysis started within an international BIPM/CCRI key comparison.. The IRMM Proton Recoil Telescope (PRT) was used to measure fluences at 1.2 5.0 and 14.8MeV All measurements were successful and the calculated fluences will be reported to the campaign host laboratory, PTB, by the end of October 2001. This key comparison exercise is registered in the BIPM database at: The data acquired were evaluated and the quantity to report has been calculated. The complete evaluation scheme for PRT data obtained using the IRMM telescope has been reviewed and a new Windows based program ("Telescope") for fluence calculations has been developed.

Output Indicators and Impact

Output indicators:

- Experimental (point-wise) data delivered in EXFOR format to the OECD-NEA databank;

- Resonance parameters (obtained by R-matrix fits to the experimental data) delivered to the OECD-NEA databank;

- Evaluated data submitted to JEFF and NNDC;

- Publications in refereed journals;

- Internal reports, JEFF reports, contributions to WPEC;

- PhD theses.

- Use of IRMM data in evaluation projects;

- Citation of IRMM experimental work in publications;

- Requests for measurement campaigns at GELINA and the Van de Graaff accelerator;

- Collaborations with external research groups.
Summary of the project

The OECD and the Data Centre of IAEA both have standards subcommittees to identify needs and improvements of new and established standards and to regularly update the neutron standards data files. These standards data files are publicly accessible worldwide at no cost. They are of major interest to various disciplines in science (physics, medicine, astrophysics) and technology.
The detailed measurements requirements for neutron data standards are collected in a high priority request list that is edited by the Working Party on International Evaluation Co-operation (WPEC) of the OECD-NEA . The IRMM is a major contributor of neutron data to these databases and is the only European laboratory capable of doing so over a wide energy range.


Neutron data reference standards are the basic data sets that need continuous improvement to ensure correct measurements for reactor safety, waste minimisation/transmutation and environmental protection. This is due to the fact that the majority of basic and applied measurements in neutron physics are relative to reference standards. It is therefore essential that these standards are continuously improved and that the underlying basic physics is understood.
The IRMM's Neutron Physics unit has a unique obligation under the Treaty of Rome for the measurement of neutron data. This requires highly specialised equipment. IRMM hosts a 150MeV electron linear accelerator (GELINA) and a 7 MV Van de Graaff accelerator as well as specifically dedicated laboratories. The GELINA time-of-flight facility has the best energy resolution worldwide. This makes the IRMM the only European laboratory (and one out of 2 or 3 in the world) that is capable of producing these data over an energy range of a few meV to about 20MeV. IRMM has a long-standing expertise in this field and has attained worldwide recognition for producing top-level neutron data measurements.

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Institute for Reference Materials and Measurements
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B-2440 Geel

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