Measurement of the ﬁssion yield of 136 Cs in the 239 Pu(n th ,f) reaction.

. A recent experimental campaign performed at the LOHENGRIN spectrometer at ILL aimed at measuring the independent ﬁssion yield of 136 Cs in the 239 Pu(n th ,f) reaction. This nuclide can have an important contribution to the total dose rate coming from spent light water reactor UOX and MOX fuels. Moreover, its impact is of ﬁrst order on the uncertainty of the total dose rate calculated in speciﬁc areas of Nuclear Power Plants within accidental conditions. One of the most important sources of uncertainty is its independent ﬁssion yield. Therefore, a new measurement of its independent yield along with a rigorous uncertainty analysis was performed. Due to its low independent yield, a new measurement technique has been applied. Ions recoiling from neutron-irradiated 239 Pu target were collected by implantation into an Al foil placed inside a vacuum chamber. This foil was then transferred to a low � -ray background setup located at nearby laboratory (LPSC). The procedure was then repeated for di ↵ erent LOHENGRIN settings. Compared to the JEFF-3.1.1 library, a decrease of the ﬁssion yield of 136 Cs with a reduced uncertainty is obtained.


Introduction
The aim of the experiment was to measure the independent fission yield of 136 Cs from the 239 Pu(n th ,f) reaction. Indeed, recent works from CEA have shown that this nuclide can have an important contribution to the total dose rate and to its uncertainty in specific areas of Nuclear Power Plants within accidental conditions. 136 55C4 Cs is a so-called shielded isotope because the stable 136 54 Xe nucleus stops the beta-decay chain of more neutron rich A = 136 isobars. Therefore, in a reactor its production is only possible directly in the fission process, or by neutron capture on 135 55 Cs. The latter is produced directly by the fission process or through the decay of the isobaric chain A = 135 (see fig. 1). Concentrations of 136 Cs were calculated for a typical light water reactor with two di↵erent fuels (UOX and MOX) and a burnup of 48 GWd/t. A sensitivity analysis was performed with the DARWIN package [1]. The results are shown in table 1. One of the main parameters contributing to the total uncertainty of 136 Cs concentration in both cases, UOX and MOX, is the remaining fraction of beta-decay directly from its isomer 136m Cs. A recent dedicated experiment performed at ISOLDE is presently under analysis [2] and will answer this question enabling an important reduction of this uncertainty. Considering that the A = 135 mass yield is relatively well known for ⇤ e-mail: abdelhazize.chebboubi@cea.fr both, 235 U(n th ,f) and 239 Pu(n th ,f) reactions, the independent fission yields of 135 Te and 135 I are in fact highly anticorrelated (due to the mass yield constrain) and their uncertainties will largely cancel in the propagation. Thus, today the remaining uncertainty of 136 Cs concentration stems from the fission yield of 136 Cs in 239 Pu(n th ,f) while its yield in 235 U(n th ,f) is expected 1 to be one order of magnitude lower resulting in smaller contribution. Therefore, a new measurement of its independent yield along with a rigorous uncertainty analysis would allow the reduction of its impact on the total dose rate calculated in specific areas of Nuclear Power Plants within accidental conditions. First, a brief description of the experimental facility will be presented in Sec. 2. For this measurement, a new experimental procedure was developed and will be presented in Sec. 3. Then, the associated analysis and preliminary results will be reported in Sec. 4.

Experimental facility
This experiment was performed at the LOHENGRIN recoil mass spectrometer [3] of the Institut Laue Langevin (ILL) (see figure 2). A fissile target is placed near the core of the reactor under a thermal neutron flux about 10 14 n.cm 2 .s 1 . Fission products recoiling from the target are deflected using a dipole magnet ("main magnet")  and an electrostatic dipole ("condenser"), achieving a separation by mass to ionic charge ratio and kinetic energy to ionic charge ratio. Then, the refocusing magnet [4] can be used to increase the particle density at the experimental position 2. The particle detectors can be placed at the focal plane (or experimental position) 1 or 2, depending on the use of the RED. The experimental campaign has been performed from 3 to 14 June 2021 with a 129 µg/cm 2 239 Pu target (99.5% enrichment) covered by a 0.25 µm thin Ni foil and a 7 ⇥ 0.6 cm 2 Ti collimator of 0.4 mm thickness. This target has been produced by molecular plating [5] at the TRIGA Site of the Department of Chemistry at Johannes Gutenberg University Mainz, Germany.

Data taking
The LOHENGRIN spectrometer selects triplets of mass, ionic charge and kinetic energy (A,q,E k ). By definition, the relative fission mass yield is written: Because of the limited beam time, the energy resolution of the spectrometer and the target matter loss with thermal neutron induced reactions, some assumptions are necessary in order to assess the fission mass yield with the best precision and accuracy. The best compromise between time and precision was reached by using an ionization chamber and measuring three kinetic energy distributions (at three di↵erent ionic charge states) and one ionic charge state distribution (at one kinetic energy). In this way, three estimates of the fission mass yield were performed. A weighted average was computed by using the correlation matrix between all estimates. All details are available in [6]. Moreover, in this work we measured independent fission yields Y(A, Z). A supplemental nuclear charge measurement 2 is performed by using a vacuum chamber surrounded by two HPGe clovers (4 ⇥ 50 ⇥ 80 mm 3 each) with an absolute e ciency of about 2.1% at 1.3 MeV (for the full setup). Those detectors allow to determine the nuclidic composition and hence the nuclear charge distribution of the selected mass, based on well-known delayed -rays. To get the absolute independent yield, it is necessary to have a reference mass for which we can measure the mass number and all its isobars. In the heavy region, the A = 139 isobar is a good candidate.
Usually, a moving tape is placed inside the vacuum chamber [7]. For each ionic charge state, the tape is moved by several decimeters in order to remove the activity from previous experimental points, prevent perturbing the next measurements. Due to the low independent yield of 136 Cs, the signal is very weak in comparison to the -ray background at the measurement position. Instead of a movable tape, the mass-separated A=136 beam was implanted into a new aluminum foil for each measurement (see figure 3). The data collection for the 136 Cs independent yield measurement was carried out as follows: • Background phase: a fresh aluminum foil (20 µm) is placed inside the vacuum chamber. Isotopes, which were implanted in the vacuum chamber, are decaying. They correspond to a background, which should be subtracted in the next implantation phase. This cycle was repeated for ionic charge settings of 17+, 19-25+, 27+, 29+, and 30+ at the most probable kinetic energy of A = 136, which was 63 MeV after energy loss in the Ni foil. This energy was determined by the ionization chamber measurements. Note that usually we consider that during a short implantation phase the target burn-up does not evolve. Nevertheless, here due to the long implantation phase such an approximation is no longer valid. An internal monitoring through the most intense -rays, coming from the decay of short-lived isobars or isomers ( 136g/m I and 136m Xe) is used instead.

Analysis and results
A brief reminder of the analysis is described hereafter. All the details can be found in [6,7]. Several steps are necessary to estimate an independent fission yield with the LOHENGRIN spectrometer: • Measurement of fission mass yields with the ionization chamber for the mass numbers A = 136 139. • Determination of the absolute e ciency of the experimental setup by using Monte Carlo simulation code and calibration sources. • Analysis of spectra with the TV program [9].
• Determination of the number of decays accounting for e ciency, intensities and sum e↵ect corrections. • Background correction coming from the vacuum chamber. • Solving the Bateman equation in order to determine the contribution coming directly from the fission process. • Correction of the target burn-up.
• Check the reproducibility between LBA and LOHEN-GRIN of the 139 Ba measurement. • Combine both LBA and LOHENGRIN measurements.
• Normalize the data with all fission mass yields measured in order to assess the absolute independent fission yield.
As explained before, the specificity of this measurement is to use two experimental areas for measuring the independent fission yield. The first one is the usual experimental setup on the LOHENGRIN spectrometer. The second one is the LBA facility to measure the 136 Cs. Also, 139 Ba was used as a tracker to ensure that both LBA and LOHEN-GRIN give similar results. Figure 4 shows the result of this procedure. Except for ionic charge state 27+ for which both measurements di↵er at the 2 level, a strong compatibility is observed. This result gives confidence into the process developed and allows us to combine both LBA and LOHENGRIN measurements. Figure 5 shows the ionic charge distribution of 136 Cs measured at LBA. It is worth noting that due to the limited time, not all the ionic charge states were measured. Therefore, the ionic charge distributions were fitted in order to estimate the missing data points. Finally, table 2 shows the cumulative yield of 136 Cs in comparison with the JEFF-3.1.1 library [10]. Due to the short decay time of 136m Cs (17.2 s), it is impossible to disentangle the contribution of the isomeric state. Therefore, the presented yields correspond to the sum of the ground and isomeric states of 136 Cs. An important reduction (factor of 5) of the uncertainty is proposed. A new central value is also proposed, which agrees with the JEFF-3.1.1 library. Our result allows reducing the impact of the uncertainty coming from the fission yield in the total uncertainty of the 136 Cs concentration.

Conclusion
A recent campaign performed at the LOHENGRIN recoil mass spectrometer at the ILL, Grenoble, France, has shown that it is possible to measure fission yields previously inaccessible by classical techniques. To do so, o↵-line -ray spectrometry was performed in a shielded low background setup located at LBA at the neighboring LPSC. Preliminary results are promising. A strong decrease of the cumulative fission yield uncertainty of 136 Cs is observed. This result is relevant for total dose rate calculation in accidental situations requested by nuclear industry. This result will also be an input to the JEFF-4 library.