The origin of the elements in the universe is one of the most basic scientific questions. According to our current knowledge, the universe originated from the Big Bang, which created some light elements, such as hydrogen, helium, and a very small amount of lithium within very short time of this incredible process^[1-3]. After then, more than 80 elements are generated via a series of complicated nuclear reactions under very harsh conditions^[3]. Radioactive decay or natural nuclear reaction might also lead to formation of some elements or their isotopes, such as U to Pd^[4-6]. It is known that the ⁴⁰Ar and small amount of radio nuclides ¹⁴C are formed by constantly bombing under the high energy cosmic rays in the Earth's atmosphere^[4-6]. Of course, we can produce some of element else from target element by high energy neutron and proton bombing typically in accelerators or Tokamak^[7-10]. However, there are some mysterious questions related to the element abundance in our universe, for example, current understanding can not account for measured excess helium element abundance in our universe^[11]. Scientists found that there were excess ratios of ³He and ⁴He insubmarine hydrothermal water^[12]. The investigation of energy balance of Jupiter also seems indicate the possibility of low energy fusion reaction there^[13].

It is well known that one element or its isotope can be converted into another element or isotope by nuclear transmutation^[14]. In 1919, Rutherford reported the first artificial transmutation from nitrogen into oxygen by alpha particle bombing^[15]. In 1932, John Cockcroft and Ernest Walton fulfilled an artificial nuclear reaction by ⁷Li bombing with accelerated protons to split the Li nucleus into two alpha particles^[6]. In 1938, Otto Hahn et al. discovered artificial heavy element transmutation, ¹⁹⁷Au+n→¹⁹⁸Au, finally→²⁰⁴Ti and²⁰⁴Pb^[9]. In fact, scientists found that transmutation can be induced in "mild" reactive systems, like in metabolism processes of vegetal and animal organisms^[16], in which some elemental atoms are transmuted into the different ones while discharging energy slowly. Kervran et al. investigated potassium and calcium content variation during the growth of 840 seeds and 403 sprouts in biological systems, and found potassium may transmute into calcium during the process of seeds growing, ³⁹K + ¹p = ⁴⁰Ca +E^[17]. Some other elemental transmutations were also reported, such as sodium to magnesium (₁₁²³Na + n → ₁₁²⁴Na → ₁₂²⁴Mg + e^- + v_e^*) and manganese to iron(₂₅⁵⁵Mn + n → ₂₅⁵⁶Mn → ₂₆⁵⁶Fe + e^- + v_e*)^[17]. Those investigations imply that the elemental transmutation is essential to maintain a balance of certain elements in the biological bodies. Later on, much more reports of element transmutation by chemical or physical was appeared, for instance, the so-called cold fusion experiments which was done by Fleischmann and Pons, in which they found that trace amount of He could be produced by electrochemical method under normal conditions while excess heat was generated^[18].

We previously reported that deuterium and helium were generated from protons by low energy nuclear reaction under light irradiation^[19-23] and the transmutation of potassium element to calcium during photochemical reaction of hydrogen evolution (HER) in the presence of hydride (negative hydrogen ions—H^-, which is offered by negative hydrogen compounds such as NaBH₄, LiBH₄ and NH₃BH₃^[22]). Potassium was chosen as the target transmutation element because of the essential roles of potassium and calcium in biological bodies.

In this work, we found interesting results about the concentration variation of Ba isotopes in the presence of hydride and CsCl. Experimental results showed that the significant concentration and ratio variation of ¹³⁰Ba, ¹³²Ba, ¹³⁴Ba, ¹³⁵Ba, ¹³⁶Ba, ¹³⁷Ba and ¹³⁸Ba isotopes after very short time reaction.Especially, the changes of ¹³⁰Ba and ¹³²Ba concentration are remarkable compared with other isotopes. These results are extremely meaningful for understanding the elements evolution on the Earth, as well as providing new way to looking into the low energy nuclear reaction.

1 Experimental details

All chemicals were purchased and used without further purification, NaBH₄ (Shanghai Guangming Chemical Reagent Co., Ltd, AR, ≥ 97 %), CsCl (Kemiao Chemical Co., Ltd., AR, ≥ 99.5%), RuCl₃ (Shanghai Guangming Chemical Reagent Co., Ltd, AR, ≥ 37.3%), and BaCl₂ (Xilong Chemical Works, AR, ≥ 99.5%). Ultrapure water: 18.2 MΩ·cm^-1 at 25 ℃ (Milli-Q water, Millipore Mili-Q reference ultrapure water purification system, USA) was used in this study. The polypropylene (PP) volumetric flask was used in this work since it is chemical stableboth in the acidic and alkaline environment. The involvment of other elements can be eliminated after very careful checking.

Prior to the reaction, all PP volumetric flasks (100 mL) were recalibrated and washed several times with high pure Mill-Q water, and then they were dried at room temperature. The typical isotope trasmutation (Cs-Ba) experiment was done as follows. 50.0 mL of ultrapure water, 1.0 mL of CsCl solution (C_Cs=200 mg· kg^-1) and 2.0 mL RuCl₃ solution (20 mg·mL^-1) were added in a 100 mL PP volumetric flask. After 5 min ultrasound treatment, different content NaBH₄ was added in the mixture solution, and the reaction was operated for days until no bubbles were generated under the room temperature. After that, the mixed solution was volumed to 100 mL with Mill-Q high purity water. For the above reaction system, it was labeled as NaBH₄+CsCl+RuCl₃. The experimental results are shown in Fig. 1.

The element and isotope concentration and ratio variation before and after reaction were measured by inductive coupled plasma mass spectrometry (ICP-MS, iCAP Q, Thermo, Waltham, USA) technique. The apparatus limits of detection for Ba and Cs isotope are 0.40 and 0.05 ppt respectively. The samples for ICP-MS measurement were not further diluted prior to analysis. The every sample was detected at least three times and the average value was presented.

2 Results and discussion

ICP-MS measurement results are shown in Fig. 1a and Table 1. The reaction mixture contains 1 mL CsCl (C_Cs=200 mg·kg^-1) and 2.0 mL RuCl₃(20 mg·mL^-1) solution in 100 mL PP volumetric flask. By changing the amount of NaBH₄ that are 236.4 and 236.4 ×4 mg, respectively, we found that the Ba isotope concentration changed with the increase of negative hydrogen compound loading (NaBH₄). It should be noted that there is no additional barium source in these initial reaction mixtures.Meanwhile, the Ba isotope ratio between ¹³⁰Ba, ¹³²Ba, ¹³⁴Ba, ¹³⁵Ba, ¹³⁶Ba, ¹³⁷Ba and ¹³⁸Ba also changed after the raection, as shown in Fig. 1b. In particular, the amount of isotope ¹³⁰Ba and ¹³²Ba decrease remarkably with the increase of negative hydrogen compound loading.

To avoid misunderstanding of the experimental results, we double checked the initial Ba and Cs isotopeconcentration in NaBH₄, RuCl₃, CsCl and BaCl₂ solu- tions by ICP-MS. As results shown in Fig. 2a, the Ba isotope concentrations of ¹³²Ba, ¹³⁴Ba, ¹³⁵Ba, ¹³⁶Ba, ¹³⁷Ba and ¹³⁸Ba in NaBH₄ and CsCl solution are lower than the instrument's limit of detection. Only a trace amount ¹³⁰Ba was found in NaBH₄(1.41×10^-3 mg·kg^-1) and CsCl(2.03×10^-3 mg·kg^-1) solution respectively. The concentrations of the Ba isotopes in RuCl₃ aqueous solution are also very low, as shown in Table 2. This means that the changing of Ba isotope concentration can only be ascribed to elemental transmutation reaction in the NaBH₄+CsCl+RuCl₃ mixture. The distribution of isotope ratio in BaCl₂ aqueous solution(C_Ba=132.0×10^-3 mg·kg^-1)was also detected, and detected ¹³⁰Ba concentration in BaCl₂ is significantly lower than other isotope concentrations.

Fig. 3a shows the Ba isotope concentrations in NaBH₄+CsCl+RuCl₃ and NaBH₄+CsCl mixtures with different CsCl concentrations. We found that the Ba isotope concentrations, such as ¹³⁰Ba and ¹³²Ba, are obvious different in two reaction mixtures(with or without RuCl₃) as shown in Table 3. The results indicates that the Ru has a catalytic effect on the Ba isotope concentration change. Moreover, increasing the initial CsCl concentration, the contents of Ba isotopes also change significantly in the NaBH₄+CsCl+RuCl₃ or NaBH₄+CsCl mixture. In short, introduction of catalyst Ru and Cs are decisive for ¹³⁰Ba and ¹³²Ba isotope conversion. By comparison the Ba isotope ratio in Fig. 3b, it is confirmed that the isotope concentration distribution of Ba is closely related to Ru and Cs addition.

Does the variation of Ba isotope concentration and ratio result from interaction of Ba itself and hydride? We conducted a reaction of hydride with Ba in the absence of Cs. In NaBH₄+BaCl₂+RuCl₃ mixture, the Ba isotope concentration almost maitained similar under different hydride loading, according to results shown in Fig. 4 and Table 4. That confirmed that the concentration variation was not due to of the reaction between Ba isotopes with negative hydrogen (NaBH₄), instead, from reaction between Cs and hydride, i.e., ^mCs + H^·-→ⁿBa.

3 Conclusion

This work reported evidences of Cs to Ba transmutation in the presence of hydride compounds. Experiments identified that the concentration and isotope ratio were changed after reaction of Cs salt with hydride at room temperature. In addition, we confirm that these phenomena are closely related to Ru catalyst role. Those results imply that some of barium isotope in nature might originate from an unknown low-energy nuclear reaction between Cs and hydride under very mild conditions.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant Nos. 21673262 and 21433007), respectively.