Fluka at low energy transfer and nuclear shell model levels

Physics, Transport and Magnetic Fields
1

Versions

Please provide the used software versions.

Beagle, which incorporates FLUKA

Description

Hi,

Mathias Labonte (mlabonte@ucdavis.edu) and I are using BeAGLE to simulate exclusive incoherent photoproduction of J/psi at the Electron Ion Collider. BeAGLE simulates the high-energy part of the event, and then calls FLUKA to simulate the breakup of the excited nuclear remnants, as discussed in Phys.Rev.D 106 (2022) 1, 012007, or BeAGLE · The EIC Software Website .

For this exclusive process, BeAGLE generates the J/psi, and simulates a momentum transfer to a single nucleon in a gold or lead target. Its nuclear model accounts for short-range nuclear correlations, but not Fermi momentum. Detecting the vast majority of the nuclear excitations is vital in separating exclusive coherent and incoherent photoproduction - one of the key physics goals at the EIC. Anyway, FLUKA simulates how the recoiling nucleon excites the nucleus.

Our problem involves nuclear breakup at small momentum transfers, where the shell-model structure of the target limits the possible final states. We are studying how detectable these excitations are, so have selected focused on ^197Au excitation events with 1 or 2 photons in the final state (and no neutrons, protons or other fragments). Selecting these events, and requiring total photon energy < 80 keV leads to the following spectra for single-photon and two-photon events:

Only80KeVEvents.pdf (26.8 KB)

We do not understand where these low-energy photons come from. The lowest excited state of ^197Au is at 77 keV (and it is visible in the FLUKA output before the selection of 1 or 2 photon events). It is a longlived (1.8 nsec) state with J^\pi=1/2^+ (vs. 3/2^+ for the ground state) Production is probably suppressed here because of the small angular momentum transfer. But, how can one get a single low-energy photon that does not correspond to a shell-model transition to the ground state? What is FLUKA doing here?

-Spencer Klein

Input files

Please upload all relevant files. (FLUKA input file, Flair project file, user routines, and data files)

4

Dear Spencer,

this is an interesting question, and the answer is that FLUKA initializes the nuclear system admitting a continuous excitation energy spectrum. Eventually, in the final gamma de-excitation stage, two scenarios can apply: 1) if the excitation energy is below the first excited state, then the excitation energy is emitted as a single artificial photon, 2) if the excitation energy is above the first excited state, a first gamma is emitted to reach this level, followed by the known discrete transition to the ground state. Please note that the version of FLUKA you are using appears to be outdated, as it treats the space between levels as a continuum, possibly emitting many soft photons. In the current version (v4-5.2) you would still get gammas with artificial energies as per the above scenarios, but the spectrum would be cleaner.

Thank you,

Stefano