High energy nuclear reactions

For experiments on nuclear reactions of high energy projectiles with targets one employs normally

  • thin, mono-isotopic targets
  • mono-energetic projectiles.

Such experiments yield well-defined differential data of energy loss or cross sections for production of individual products, reactions mechanisms or secondary particles. Most differential nuclear reactions are very well described by nuclear models and experimental results can be reliably predicted.

Modelling of nuclear reactions in thick targets is more complicated because projectiles loose energy through elastic and inelastic interactions and they can be slowed down even below the reaction barrier; however, there are reliable model descriptions for thick target experiments as well.

Experimentalists will normally avoid experiments in which very high energy projectiles hit very thick targets because secondary particles and nuclear fragments can again make nuclear reactions. This cascading of reaction products and consecutive reactions makes the modelling of experimental results very complicated.

Beispiel-Reaktion

The field of nuclear reactions of very high energy projectiles with very thick tartgets is the scope of our own experiments. Projectiles having energies of GeV per nucleon interact with targets having several centimeters or decimeters thickness. In these experiments one measures integral data which come from reactions of the primary projectile as well as of projectile fragments and secondary products that were formed in the primary interaction. Depending on the energy of the primary projectile one may also have ternary or higher order reactions. From first principles one should be able to calculated integral data with Monte-Carlo methods using well tested nuclear models which are used for differential data. One just has to consider all projectile, fragment and product reactions that can happen in the thick target.
As this type of nuclear reactions is avoided by experimentalists one has only few data available for comparison with calculations.

In the figure the intensity distribution of the reaction product 140La from the nuclear reaction 139La(n,γ)140La along a 50-cm long Pb target of 8 cm diameter and coated with 6 cm paraffin is shown. Red points are from a model calculation (absolute values).

The model calculation is for the reaction of 2 GeV protons impinging into a lead target of 8 cm diameter and 50 cm length which is surrounded by 6 cm of paraffin. Neutrons produced in the target make secondary reactions and are partially moderated in the paraffin. There are lanthanum samples distributed on top of the paraffin in which the 139La(n,γ)140La reaction takes place. The figure shows the absolute amount of 140La produced in the samples. There is remarkable agreement between experimental results and calculation showing that even complex integral data can be reliably calculated with modern nuclear models. The mean deviation between experiment and calculation is only ±2%.

This positive agreement between eperiment and calculation disappears when one deals with very high energy reactions. In these reactions one finds that the iintegral production cross section for many nuclides cannot be reproduced and the experimentally determined amount of produced neutrons is much higher than predicted by the models. Moreover one finds that some reaction products come predominantly from secondary reactions whereas they should be much more effectively produced by the primary projectile. An extensive description of these unresolved findings is presented in the publication:   W. Westmeier et al., „Correlations in Nuclear Interactions between Ecm/u and Unexplained Experimental Observables“, World Journal of Nuclear Science and Technology 2 (2012) 125-132, where a limiting energy is proposed above which unexplained events are observed.

Data presented in the above paper stem from radiochemical surveys in which production cross section and neutron densities are determined with high accuracy. Similar discrepancy between experimental data and common understanding have been found in data measaured with emulsion detectors. In these detectors one can see and measure the tracks from charged reaction products. From observed tracks and calibration data one can deduce that there are distinctly different temperature regimes in projectile nuclei whereas target nuclei always have the same (low) temperature. Details of the data and findings are published in:
E. Ganssauge et al., „Potential Correlations between Unexplained Experimental Observables and Hot Projectile-Like Fragments in Primary Interactions above ECM/u ≈ 150 MeV”, World Journal of Nuclear Science and Technology 3 (2013) 155-161.

Furter investigation of nuclear reactions of very high energy projectiles with emulsion reveal two different types of interaction which are termed “Spallation” and “Burst”.  Whereas spallation reactions are well understood with current models the results from Burst interactions are unexplained. It seems, however, that Bursts and other unexplained phenomena as described in the paper above may originate from the same origin. Results are published in: R. Brandt et al., „Two Ways of High-Energy Heavy Ion Interactions: Spallation and Burst“, World Journal of Nuclear Science and Technology 5 (2015) 73-87.

More details on BURST reactions as well as an attempt to calculate a reaction with the MCNPX 2.7 Monte-Carlo program have been published in:  R. Hashemi-Nezhad et al., “Further Studies of BURSTS and Spallation in High-Energy Heavy Ion Reactions”, World Journal of Nuclear Science and Technology 7 (2017) 35 – 57

We wish to repeat our proposal from the paper from year 2012 that one should make an experiment in which the relevant energy range around Ecm/u ~ 150 MeV is scanned thoroughly in order to determine the transition energy very precisely. One can the predict when such unexplained events will occur. This would enable a geared search for the origin of these unexplained results. The proposed experiment is an irradiation of 5 cm thick copper disks with carbon-12 projectiles or an equivalent projectile/target combination.