Eur. Phys. J. A (2026) 62: 61
https://doi.org/10.1140/epja/s10050-026-01827-2

Regular Article - Experimental Physics

Time-dependent $\gamma$-ray emission after spontaneous fission of $_{}^{252}$Cf with an experimental verification

¹ European Commission, Joint Research Centre (JRC), Retieseweg 111, 2440, Geel, Belgium
² CEA/DES/IRESNE/DER/SPRC/LEPh, Cadarache, 13108, Saint-Paul lez Durance, France
³ Extreme Light Infrastructure—Nuclear Physics, “Horia Hulubei” National Institute for R&D in Physics and Nuclear Engineering, 30 Reactorului Street, 077125, Magurele, Romania
⁴ Subatech, Nantes University, CNRS/IN2P3, IMT-Atlantique, 44307, Nantes, France

^a This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 July 2025
Accepted: 24 February 2026
Published online: 24 March 2026

Abstract

The time-dependent emission of prompt fission $\gamma$ rays was measured at JRC-GEEL with the VESPA spectrometer, the VErsatile $\gamma$ SPectrometer Array, coupled to a position-sensitive fission detector. To be able to do so, we developed a method to assess and correct for $\gamma$-rays induced by scattered prompt fission neutrons. This research follows measurements performed in the 1970 s; after that, no more experimental work was performed until about the past decade. The close geometry of $\gamma$-ray detectors and the fission detector was the key to extracting the time-dependent fission $\gamma$-ray spectra. In addition to the measurements, model calculations with the Hauser-Feshbach Monte-Carlo code FIFRELIN were performed. These calculations can very well confirm the observed trend. The results show for the first time that late fission $\gamma$-ray emission occurs well beyond 5 $\mu$s after the instant of fission. Our new data suggests that about 1.5% of the fission $\gamma$ rays are still emitted later than those 5 $\mu$s after fission. In conclusion, for $_{}^{252}$Cf(sf) we find a total average fission $\gamma$-ray multiplicity of ${\overline{M}}_{\gamma ,\mathrm{tot}}$ = 9.4 ± (0.2$_{\text{stat}}^{}$ + 0.4$_{\text{syst}}^{}$), with a late multiplicity ${\overline{M}}_{\gamma ,\mathrm{late}}$(t > 2.5 ns) = 0.70 ± (0.22$_{\text{stat}}^{}$ + 0.57$_{\text{syst}}^{}$), corresponding to (7.4 ± 1.6$_{\text{stat}}^{}$ ± 4.2$_{\text{syst}}^{}$)%.