A review of experiments reporting non-conventional phenomena in nuclear matter aiming at identifying common features in view of possible interpretation

The purpose of the present paper is to clarify, as far as it is possible, the overall picture of experimental results in the field of non-conventional phenomena in nuclear matter published in scientific literature, accumulated in the last decades and still missing a widely accepted interpretation. While completeness of the collection of the experiments is not among the aims of the effort, focus is put on adopting a more comprehensive and integral approach through the analysis of the different experimental layouts and the different results, searching for common features and analogous factual outcomes in order to obtain a consistent reading of a lot of experimental evidences that appear, until now, lacking a classification in a logic catalogue which might be compared to a sort of building and not to a collection of single stones. Particular attention is put on the issue of reproducibility of experiments and on the reasons why such a limitation is a frequent characteristic of many experimental activities reported in published papers. This approach is innovative as compared with those already available in the scientific literature. In a synoptical table a comprehensive classification is given of the twenty experiments examined in terms of type of evidences that are ascertained by the experimenters in their published papers but are unexpected according to well established physical theories. Examples of such unexpected evidences (named also non-conventional or weird) are: excess heat generation, isotope production, reduction of radioactivity levels, and production of neutrons or alpha particles.

In a synoptical table a comprehensive classification is given of the twenty experiments examined in terms of type of evidences that are ascertained by the experimenters in their published papers but are "unexpected" according to well established physical theories.Examples of such unexpected evidences (named also non-conventional or weird) are: excess heat generation, isotope production, reduction of radioactivity levels, and production of neutrons or alpha particles.These evidences are classified taking into account both the material where the evidence takes place (Solutions, Metals, Rocks and Artificial materials) and the stimulation tecniques (supply of electric voltage, irradiation by photons, mechanical pressure) used to generate the evidences (which do not appear in absence of such stimuli at an appropriate intensity. As an Appendix, "identity cards" are provided for each experiment examined, including details emerged during the experiment and reported in each pertaining paper, that sometimes are not given adequate consideration neither by the author of the experiment nor in other review papers.The analysis of the details provides suggestions (also referred to as clues in this papers) used to formulate the content of the second part of each identity card where inferences deduced from facts are outlined in view of presenting tentative interpretation at microscopic level.This is done by concentrating attention on the clues repeated in different experiments in order to yield possible explanations of the "unexpected" evidences.
The main outcome of such analysis is that, in all examined cases, a common "operation" can be identified: the stimulation techniques mentioned above can be interpreted as a sort of compression 1 producing a ramp of energy densification (with reference to volumes in space, or time coordinates).Five types of densifications were identified.This reading in terms of energy densification is in accordance with the previsions of the Deformed Space Time theory, reported in scientific literature, in the contest of a generalization of the Einstein relativity theory, according to which the existence of energy thresholds is found to separate, for each interaction, the flat metric part from the deformed metric part and the appearance of new microscopic effects as a consequence of trespassing such thresholds.The phenomena occurring in the deformed part of the interaction metric are governed by the energy

Early phase
It is well-known that the issue of Low-Energy Nuclear Reactions (LENR) 2 , has a complex, controversial history, since the original work in 1989 [1] which generated diffused expectations for the feasibility, by taking advantage of previously unknown nuclear phenomena, of energy production at low cost and without negative side effects.The first phase was characterized by a huge amount of attempts in the scientific community and in industrial companies but It led to a disappointing early dismissal by the majority of the scientific research community; a typical example of criticism is the paper by Huizenga published in 1993 [2].

Three decades of experimental efforts
The LENR experiments conducted later, an effort which nevertheless unavoidably continued for decades in various parts of the world -a list of papers can be found in Ref. [3] -never were recognized to have achieved the high level of rigor and repeatability, which characterizes accepted modern science.
Among the most cited reviews are those published in the years 2007 to 2016 by two authors: Edmund Storms [4], [5] and Steven B. Krivit [6], [7], [8] both very skeptic about the quality of the results obtained by the experimenters engaged in this field.Nevertheless, activities on this subject continued.Three updated and extended reviews have been recently published.In a paper published on Nature in 2019 [9] C. P. Berlinguette and others recognize that their "efforts, have yet to yield any evidence of cold fusion".Nevertheless, they "believe that there is exciting new science to be done within the parameter space of cold fusion experiments, and that this is an area worthy of engagement from the broader scientific community, even if the discovery of cold fusion at high enough rates for energy applications does not materialize.In the review paper by Nagel [10] besides references of several experimental papers, some classification of types of experiments is also given.The main limitation is that only experiments involving deuterium or hydrogen interacting with palladium or nickel are concerned.The most recent review (December 2021) is in the paper by L. O. Freire and D. Andres de Andrade [11] who, after listing a large share of the experiments published in literature, avoided to 2 Since 1989 for three decades, the anomalous transformations of matter have gone under multiple names that have increased confusion concerning phenomena that are already complex to understand; we mention a few frequently used names: Cold fusion, Low-Energy Nuclear Reaction (LENR), Condensed Matter Nuclear Physics, Nuclear metamorphosis.Piezonuclear reactions was also used as a consequence of experiments where unexpected evidences occurred when pressure increase was involved.We chose the last one which seemed to us less equivocal and more responsive to the need to give a genuinely new name to a genuinely new phenomenon.Indeed, a series of experimental results on transformation of matter which are well documented in scientific literature give rise to undeniable contradictions with established physical theories and have not been interpreted yet through models that have reached a widespread consensus.
formulate any sort of conclusion and closed their paper with two entirely open questions: the first "whether the golden dream of the fusion is approaching our time; the second, whether this is a dream or a nightmare.In case cold fusion becomes a reality, perhaps the answer to the second depends on the time that technology development starts".
Of some interest is also a list of papers named Synopsis of Refereed Publications on Condensed Matter Nuclear Reactions (2019) [12] which is more a bibliographical list than a synopsis since for each paper only journal, title, authors, and affiliations are reported, with very limited indications of the results obtained and no detailed discussion.A site named LENR-CANR [3] (already mentioned) features a library of papers on LENR including more than 1,900 original scientific papers.The papers are linked to a bibliography of over 4,500 journal papers, news articles and books about LENR.
It's worth mentioning initiatives dealing with LENR undertaken by official research Agencies at national and international level.
The most engaging effort is the one conducted under the auspices of the European Commission [13] through a Program named Clean Energy from Hydrogen-Metal Systems [14] responding to the call for proposals dealing with the objective Developing a new source of clean energy.The program started in 2020 with a duration of 4 years and a budget of 5.5 million euros.A possible extension due to the delays imposed by the COVID pandemic is under discussion.In execution of this program a large number of papers have been published and are being published in scientific journals including Journal of Condensed Matter Nuclear Science [15], specialized on this theme, to which an international Conference is dedicated annually since more than twenty years [16].Discussion on the possible prosecution of this effort are still pending.
As an example of attention paid also recently on the issue of LENR within qualified scientific institutions, one might mention a paper [17], published by NASA in 2020, dealing with "novel" nuclear reactions observed in bremsstrahlung-irradiated deuterated metals.
Attention to the issue of LENR has been repetitively paid by US institutions in the realm of defense research activities under the responsibility of DARPA.A hearing on the subject of LENR, was held in 2016 on request of the House Committee on Armed Services (US Congress) organized by the Department of Defense.The outcomes of the hearing, reported in [18], focused on two methods considered as more extensively addressed by researchers: muon catalytic fusion and electrolysis.As a consequence of the political discussion the foundation of ARPA-E was established.This effort has not led until know to conclusions on the issue of LENR: from the experimental viewpoint unpredicted events of different type continue to be observed, but reproducibility is poor and a convincing explanation of the phenomena is missing even at a phenomenological level.

Proposals formulated for theoretical interpretations
Before dealing with further developments on LENR it's worth wile to formulate a few remarks on the situation of the theoretical considerations on the issue of LENR.
A large number of theoretical models for LENR are commented by V. A. Chechin, et alii in their paper dated 1994 entitled Critical review of theoretical models for anomalous effects in deuterated metals [19].The major limit of this review is that only interpretations dealing with deuterium are taken into consideration.A more recent (started in 2012) collection of papers dealing with Low Energy Nuclear Reactions can be found in the site New Energy Times under Low-Energy Nuclear Reactions (LENR) Theory Index [20] where links are given to a variety of specific papers and mention is made to a categorization of the theories in several classes, suggested by V. A. Kirkinskii, Yu. A. Novikov "Theoretical modelling of Cold Fusion", Novosibirsk 2002, cited as Reference 9 in Reference [21].
Among the approaches suggested for theoretical interpretation of the experimental results mention should be made of a theoretical model [22] (named MFP after the proponents Mignani, Francaviglia, Pessa) based on the Deformed Minkowski Space also named Deformed Spacetime -DST.This model has been applied by us in a recent paper [23] (published in Symmetry) comparing its predictions with a limited set of experimental data available in literature that were considered to be more accurately reported in the corresponding papers: the predictions have resulted to be coherent with measured outcomes.
Even if it is undeniable that the issue of LENR is continuing to be given widespread attention, it must be recognized that the picture emerging from the review engagement deployed until now is far from being clear at phenomenological level (even reproducibility is lacking in many cases) and that attempts to deepen interpretation of experimental evidences and to indicate a theoretical explanation are very diversified and even controversial.Such a situation is to be called an open question that should not be left as such.

A new strategic approach adopted in USA
Recently ARPA-E addressed again the issue of LENR after the non-enthusiastic conclusion of the Hearing at US Congress level held in 2016.A workshop was organized in 2021 [24] with the purpose to "explore compelling R&D opportunities in Low-Energy Nuclear Reactions (LENR), in support of developing metrics for a potential ARPA-E R&D program in LENR.Despite a large body of empirical evidence for LENR that has been reported internationally over the past 30+ years in both published and unpublished materials, as well as multiple books, there still does not exist a widely accepted, on-demand, repeatable LENR experiment nor a sound theoretical basis.This has led to a stalemate where adequate funding is not accessible to establish irrefutable evidence and understanding of LENR, and lack of the latter precludes the field from accessing adequate funding."Subsequently ARPA-E asked [25] "outstanding scientists and engineers from different organization, scientific disciplines, and technology sectors to participate in an Exploratory Topic (dealing with LENR both on scientific aspects and on implementation capabilities).Multidisciplinary and cross-sector collaboration spanning organizational boundaries enables and accelerates the achievement of scientific and technological outcomes that were previously viewed as extremely difficult, if not impossible.On September 2022 a call for proposal [26] was issued by ARPA-E and on February 2023 the selection of 8 project teams was decided for a total funding of 10 million dollars [27]. It's interesting to read the synthetic motivation for this effort published by ARPA: "The teams announced today are set out to answer the question 'does this area show promise, and if so, how?Or can we conclusively show that it does not?'While others have shied away from this space, ARPA-E wants to break through the knowledge impasse and deepen our understanding." A commented chronicle of the steps undertaken by ARPA-E has been published by ANS (American Nuclear Society) [28] under the title "ARPA-E picks eight teams to prove -or debunk -low-energy nuclear reactions".

The contribution by the present paper: a collection of experimental data organized in a coherent taxonomy to identify common features in view of possible interpretation.
The actions enforced by ARPA-E in 2021 aroused our interest towards reconsidering with a new comprehensive approach to experimental data this longstanding issue that we have addressed almost at the same time with the paper published in Symmetry [23], already mentioned, where a theory based view point has been adopted with the discussion of experimental data limited to a small number of experiments.
The present paper, on the contrary, is not devoted to present a theoretical formalism, but to the systematic reconnaissance, deepening and classification of experimental evidences published by different authors.Most of the published papers adopt a fragmented and restricted approach in the sense not only that each one is concentrated on the single experimental layout utilized, but also that the results are very often presented as a confirmation of a theoretical interpretation suggested by that group of researchers specifically for their own results.Consequently, we considered useful, having put temporarily aside the theoretical assumptions adhering to which any single experiment was accomplished, to provide a synopsis in view of: -commenting on the degree of fulfillment of high standards of design and implementation of the experiment, on the completeness of the details exposed and the extension of reproducibility achieved -commenting on possible improvements of reproducibility -identifying the possible presence of features common to several experiments that, beyond the details of each single experiment, may help in identifying some common action mechanisms to be considered shared among the different experimental approaches adopted.
Hence, the basic idea of the present paper, is first of all, the identification of a typology of cases each of them found representative of a variety of hundreds of experiments published in journals or in international congresses using the material environment where the experiment is conducted as a primary classification logic.The second step is the examination for each emblematic case, of a series of features as described in the following paragraph.
This work of collection, description, classification and comment of experiments already conducted may be a background providing support for the most valuable design (and possibly interpretation) of the next experiments promoted by the ARPA-E initiative mentioned above.
We are fully convinced that new experiments are necessary to exit from a sort of "suspension of judgment" on the issue of LENR; the outcome of the new experiments to be performed should be either to confirm, explain and possibly expand the indications acquired until now or to deny with convincing arguments (in primis through ad hoc experiments, in accordance with Popper's falsification principle) the credibility of the phenomena widely reported but until now controversial.To put the issue synthetically using old Greek words aporìa (contradiction) has generated epoché (suspension of judgment).The suspension should not last indefinitely (the question was raised more than 30 years ago with the claims by Fleishman and Ponds and it is still pending); exegesis (explanation and clarification) should be achieved promptly.
Moreover, we give indications in the direction of identifying the densification of energy (in all its different concrete realizations involving space and/or time dimensions) as a possible unifying concept applicable to all the emblematic cases considered.
In this sense, the conclusions reached in our previous article [23], already mentioned, where we indicated that the DST theory [22] allows to predict the outcomes resulting from a limited set of experiments, are extended to all the experiments examined in the present paper.This result can be considered a step forward overcoming a situation characterized by the circumstance that different theoretical models are in the available literature used for different experimental results.
We can therefore say, from the experimental results, that this theory provides a single and unambiguous interpretation for all anomalous nuclear physics results in all unconventional experiments.The unifying concept turns out to be the densification of energy in all its different concrete realizations.On the contrary, Authors of other experiments -and their related theoretical explanations -have always proposed phenomenological-level modeling hypotheses to interpret at least one experimental case without, however, being applicable to at least one other, different experimental case.
Our contribution aims to avoiding that the heritage associated with a wide engagement deployed for LENR during a long time may be disregarded or wasted both in terms of specific data gathered and in terms of methodologies acquired part of which might be utilized again with proper modifications.

CHARACTERISTICS EXAMINED FOR THE CLASSIFICATION OF THE EXPERIMENTS
For the classification of the experiments the following three characteristics are examined: -the type of observed experimental evidence which is considered "weird" (in the sense of being unexpected or even considered impossible according to generally accepted physical theories -anomalous being a synonym of "weird" in this respect) and the instrumentation adopted to this purpose -the material matrix in which the evidence takes place -the modalities through which the onset of the evidence is stimulated.
The evidence can be either microscopic (for instance detection of nuclei or particles not present before the onset of the phenomenon 3 ) and/or macroscopic (such as either excess energy production in form of heat or pressure generated or visible modification in the appearance and structure of materials present in the experimental apparatus).
We call "experimental approach" the combination of item b. and item c. (for instance palladium electrodes in an electrolytic cell where voltage is used to stimulate the evidence expected).
The use of this classification is not only taxonomic.Hence suggestions can be obtained, in planning and performing further experimental activities, pertaining to the type of events to be searched and detected as well as to choices on the selection of instrumentation to be included in the experimental apparatus and even on the geometrical disposition of single detectors, since anisotropy characterizes the deployment of events resulting from this type of experiments.We underline that anisotropy of experimental outcomes can be the cause for lack of confirmation when a successful experiment is duplicated by a different group since minor differences in the layout of the experiment (for instance placement of detectors) my lead to miss the outcome looked for.

FIELD OF INVESTIGATION
The present synopsis is concentrated on experiments where transformation of matter is implied.Only "Nuclear non conventional phenomena" are considered while elsewhere (see for instance [23]) also non nuclear phenomena are addressed.
Remaining in this specific field, we notice that the labelling "Low Energy Nuclear Reactions -LENR", is to some extent confusing since the expression "Nuclear reactions" is well established in physics to refer to the outcomes of nuclear forces as described by the Standard Model in the framework of which, the type of phenomena under investigation here are theroretically foreseen to be impossible.The expression "Nuclear non conventional phenomena" pregerred by us keeps the term nuclear since the modifications detected pertain to the nucleus, but drops the word "reaction" to be reserved to the consequences of the intervention of nuclear forces as described by the Standard Model.As a shortcut of "Nuclear non conventional phenomena" we recommend the use of "Nuclear metamorphosis" as well.It shoud be noticed that in physics the widest term ( i. e. the most generic) used in such situations is "interaction".
In the present work we do not consider the following two cases: catalytic nuclear fusion from muons 4 (mesic atom method) and ablation-catalyzed nuclear fusion generated by laser beams 5 .In fact, both cases do not encounter anomalies and are well encoded in the context of well-known applications of nonrelativistic quantum mechanics at the nuclear level (nuclear wave function superposition and consequent increase of tunnel probability).
The principal selection criteria have been semplicity of the layout and readability of the data describing both the experimental set up and the results substatiating the phenomenon under investigation.As already hinted, the main reasons for this choice have been on one side favouring duplication of experiments by other researchers, on the other side facilitating the identification of essential features common to the different experiments that could be useful to enlighten their nature.

TERMINOLOGY ADOPTED TO QUALIFY THE EVIDENCES REPORTED
As the title of this section underlines, the attention is focused on experimental evidences.By this term we mean what is found, highlighted, recorded and -that is decisive -measured in the experiment described in each paper; for instance, emission of 7.
particles quantitatively measured by means of adequate instrumentation properly operated and calibrated.The term evidence, to be interpreted as referring to a specific occasion, is not to be confused with the term phenomenon which refers to a systematic repetition of coherent evidences in given conditions.
Since the different papers examined here are not based on a unique coherent terminology, it is considered useful to adopt the terminology described in the following table [33] to create a sort of hierarchy among situations with different level of reliability as a consequence of: -sporadic or systematic occurrence -degree of governability of the physical process characterizing the experiments -multiplicity of research teams where experiments have been successful.

Sporadic evidence
Measurements indicate that some events take place (for instance the detecion of a particle) but the experimenter does not know when this may happen or not (i.e. control parameters are not identified clearly).
Even the recipe of what details may lead to the events looked for, is not clear.

Reproducible evidence
The

Interpretation of a phenomenon
Identification of well established physics-defined objects intervening and of their mutual interactions.

Explanation of a phenomenon
Recurring to the asset of knowledge dealing with more general phenomena and laws, which can be used as a justification of the new phenomenon encountered.These knowledge can in some case be only hypothesized and expecting confirmation or refutation In synthesis, a set of experimental evidences must, as a prerequisite, be compliant with the generic concept of reproducibility 6to be accepted as a proof of the existence of a physical phenomenon that is agreed to exist in nature."Ideally, an experiment or analysis should be described in sufficient detail, so that other scientists with sufficient skills and means can follow the steps described in a published work and obtain the same results within the margins of experimental error" [33].8.

CHARACTERISTICS OF THE EVIDENCES RESULTING FROM THE EXPERIMENTS EXAMINED
The experimental cases examined are, first of of all, classified according to the material where the evidence takes place: -solutions in gaseous or liquid environment -metals of different composition and in different shape -rocks and artificial chemical materials such as sequioxane.

Types of evidences encountered
A distinction is introduced between: -microscopic evidences consisting in: detection of neutrons or alpha particles not initially present; detection of nuclei not initially present; reduction of the quantity of an isotope initially present; reduction of radioactivity levels (gamma rays directly arising from a metamorphosis have not been detected, up to now.
-macroscpic evidences based on the appearance of excess heat production and/or localized deformation of components of the experimental apparatus and/or modifications of radioactivity of a sample;

Identification of techniques used to stimulate the onset of the evidences
The evidences appear only in the presence of actions that modify the conditions characterizing the environment.These actions can be classified as follows: -compression via electricity -compression via photons -compression via ultrasounds with onset of cavitation -compression via gas injection -compression directly by shear (mechanical compression).
As anticipated in footnote 1 we use the term "compression" to indicate the operation activated by the experimenter; as such it is objective.We consider energy densification an inference of possible consequences of the operation on the status of the system.

COLLECTION OF THE INFORMATION PERTAINING TO THE EXPERIMENTS CONSIDERED
The information and the subsequent considerations formulated in this paper rely on a thorough examinations of the most significant information available in literature for each type of experiment that has provided evidences interpreted by the experimenters as a proof of the existence of phenomena in matter involving nucleus that can be called "weird" i.e. lacking until now widely accepted interpretation and explanation (in several cases the use of the term phenomenon is questionable due to weakness in repeatability, see footnote 6 ).

SYNTHESIS DESCRIBING TYPES OF EXPERIMENTS AND EVIDENCES REPORTED
In the following, an overall synthesis table is provided.For each experiment, the evidences detected are summarized in correspondence with the stimulation techniques used to generate such evidences.It should be noted that the meaning of the last column dealing with the modality leading to the energy densification will be explained later (see Sec. 3.3).In the first 9.
column of the tables reported below, the natural numbers, possibly followed by a letter, from 1. to 17, refer to the corresponding sheets in the Appendix. 10. 11.

DETAILED INFORMATION COLLECTED FOR EACH EXPERIMENT
In the Appendix, for each experiment examined, a descriptive identity card it is provided; this card is divided in two sections named "objective results" and "inferences" respectively.

"Objective" information for each experiment
The first set of information can be considered as "objective" which means that it was ascertained by the experimenters (it includes: what materials were present, what procedure were adopted and what was found, depending on what was looked for and on what instruments were used).Attention is focused on: -Material where the evidence takes place It should be stressed the issue of "what was looked for" and of "what instruments were used": for instance, in some cases, is presumable, in particular by comparison with the results of comparable experiments, that radiation could have been released but it was not detected because either it was not expected and consequently not looked for, or expected but not found because the necessary detection system was not installed.

Inferences for each experiment deriving from the interpretation of evidences
The second set of information, to be considered as inferences (in other words deductions, interpretations) formulated by us, includes: -Estimated degree of description completeness and of reproducibility level -Interaction environment -Interaction agents -Modality for energy densification -Phenomenon type -Microphysics Interpretation The evaluation grid of repeatability of each experiment here introduced is not finalized to a judgment on the value of the paper, but only an indication of the viability of repetition with the available information.A four-value scale is used: Improvable, Sufficient, Good, Very good.
Interaction agent is the object, present in the apparatus, which, according to the interpretation adopted, participates to the onset of the events take place, eventually in synergy with secondary participants.Interaction Environment indicates where the main Interaction agent is located and where potential secondary agents come from.
Experiments providing microscopic evidences of Nuclear Metamorphosis (concerning element(s) production or variation from one starting element to another or isotopic variations that occur in an appropriate and appropriately stressed environment without the use of radiation) 7 are categorized as follows: 12.
(Reduction of radioactivity) 8From a substance containing at least one radioactive element or one radionuclide stable elements are produced as a result of transformation of the starting radionuclides and reduction of the activity level of the starting substance from that before the treatment.Possible application area: Removal of radioactivity from nuclear waste.

Isotopic change
From a multi-isotopic starting element, stable isotopes are produced with alteration of the natural isotopic abundances.Possible application area: Removal of radioactivity from nuclear waste as demonstrated in experiments conducted with Thorium and Nickel (see. in the Appendix experiment type 4 and 5 respectively).

Production of elements.
From a stable starting element, stable elements previously not present in the sample (normally lighter than the starting one) are produced Possible application area: Production of valuable elements, for example rare earths or helium.
Nuclear emissions 9Occurrence of neutrons or α particles that are NOT α radiation from nuclear decay.Possible application area: Neutron or α particle source productions; power generation.
This classification aims also at justifying the use of the term phenomenon (see 2.2) since a certain type of evidences are repeated in different conditions.
Microscopic interpretation is a tentative description of the phenomenon in terms of interactions among well-established physical objects, such as particles and nuclei.
In the analysis of the literature, additional information, besides what is listed above, was acquired, when available, dealing with: experimental conduct ("how it was proceeded" also in view of reproducibility considering the parameters influencing the outcome the possibility of adjusting and controlling the process) -additional outcomes during the experiments.
This information contributes, as source of clues, to substantiate the inferences deduced.Among the additional outcomes one should mention: -Production of air bubbles By inter-comparing the occurring of these clues, suggestions can also be derived about additional instrumentation to be used to detect other evidences in further, hopefully conclusive experiments.

13.
Even though, in many cases, the additional data mentioned above have been valuable to provide clues to formulate the inferences, they were not reported in the data sheets for lack of space.Interested readers may have access to these data looking at the References mentioned for each experiment.
We underline that, for each experiment type, three different levels of details are made available, corresponding respectively to: the synthesis presented in section 3.1 -the dedicated data sheet reported in Appendix the bibliographic references to the original papers describing each experiment mentioned in each data sheet.

The process of energy densification
We call energy densification the process of reaching energy densities that are critical -in space and time -for the occurrence of the phenomenon 10 , which may have entirely new and peculiar characteristics, even to the point of apparently contradicting well-known and acquired laws, such as, for example, the conservation of total energy and the second principle of thermodynamics.But such violations are only apparent and are resolved in the context of DST theory [23] [22], which treats spacetime as an energy-dependent elastic medium and contextually energy reservoir, without invoking any concept involving the vacuum state.
The existence of energy thresholds is found to separate, for each interaction, the flat metric part from the deformed metric part.The phenomena occurring in the deformed part of the interaction metric are governed by the energy density in the spacetime (volume [34] and time interval [35] and Chap.16, par.16.3.1 p. 242 -245, par.16.3.5p. 249 -251 of [22]).
From the mathematical standpoint, the energy E has to be considered as a dynamic variable, because it specifies the dynamic behavior of the process under consideration, thus providing, through the metric coefficients, a dynamic map in the energy range of interest of the interaction ruling a given process, with regards to the energy density in the space volume [34].At the experimental and practical level, the outcome of energy densification results in reaching the values of critical pressure, i.e., energy per unit volume, and critical power, i.e., energy in the interval of time defined as unit, at which the phenomenon is triggered and produces measurable effects.With respect to the energy density in the time variable, the power density for piezonuclear reactions was also estimated, as an order of magnitude, starting from the DST theory [35].

Modes to obtain stimulation in different configurations
In particular, in the case of a liquid medium, densification occurs by employing ultrasound to induce critical conditions in phase space with a mechanism that can be traced back to cavitation, 11 which gives conditions for energy thickening.In metals, it can be imagined that the confinement function can be fulfilled by Ridolfi cavities and the ultrasonic wave can be generated by fracture propagation 12 .In rocks, it can be imagined that Ridolfi cavities would intervene, but only in the presence of brittle fracture and subsequent pressure wave.More on this subject is set forth in reference [21] chapter 16 paragraph 16.3 pages 242-251.There it is reported that the orders of magnitude of cell sizes 13 are those in the range of (4 to 8) microns for space and microsecond for time respectively. 10The label "quadridimensional densification" is suggested to express that both energy density in the space volume and energy density in the time interval must be considered. 11Cavitation arises when a plane wave of pressure of given wavelength impinges on a gaseous bubble internal to matter if the diameter of the bubble is much less than the wavelength: by Pascal's principle it produces a spherical-symmetric collapse in that bubble.
In the case of a gas pressure on metal powders, it is the gas pressure that acts directly on the matter producing the acceleration in phase space that leads to the critical cell and thus to the conditions of nuclear metamorphosis of matter.
In the case of a massive material subjected to hydrogen loading, mechanical stresses arise.They can change the microstructure of the material until local microfractures develop.The latter can occasionally give rise to an accelerated increase of the pressure, thus leading to the microcavity reaching critical conditions, and hence to the nuclear metamorphosis occurring into corresponding the phase space cell.Since the development of such local fractures is occasional and cannot be linked in a predictable way with hydrogen loading, the critical conditions in phase space are difficult to reproduce.
For a solid material directly subjected to mechanical stress, up to a critical load, its overcoming by fracture is produced.In the event that the fracture is "brittle" there is an acceleration leading to nuclear metamorphosis into the phase space inside the critical cell.Should the fracture be "ductile", the acceleration is insufficient and never reaches the critical conditions in the cell, needed to nuclear metamorphosis.
To the purpose of obtaining an energy densification method useful to generate phenomena of nuclear metamorphosis in condensed matter, in particular in solid matter, it's possible to consider also the class of processes consisting in "loading" light elements into the bulk of a solid matrix also of crystalline type; this loading process has been widely used in experiments conducted in the framework of experimental activities dealing with the so called "cold fusion" for instance using hydrogen or deuterium as gases to be introduced in a metal bulk material.Nevertheless, such a process is highly unpredictable with respect to the aim of reaching the critical energy density necessary to obtain nuclear metamorphosis.Even if the deformation of the material undergoing loading can supply clues on the energy densification process, any single loading operation has its own dynamic and is characterized by its own history that do not necessarily ensure a highly reliable reproducibility when targeting the onset of nuclear metamorphosis; this is shown by the discording results to be registered in experimental layouts that appear to be overlapping.

ISSUES AND TOPICS SHOWING WEAKNESSES IN THE EXECUTION OF RESEARCH PROGRAMS Difficulties in replicating in other laboratories has been the Achilles' heel of this research area
As already noticed, a problem displayed by the existing literature on this topic, is that the experimenter himself sometimes fails to reproduce what he has achieved.Moreover, when the experiment is repeated elsewhere there are difficulties in obtaining results coherent with the outcomes of the original experiment (see section 2.2 above).This can be due to the adoption of experimental procedures which are incorrect with respect to those adopted in the experiment to be repeated.
An important aspect, to be kept in mind, is that in the designing and performing the considered experiments, there has been a remarkable lack of modeling support, from which to draw guidance on what to look for.Additional problems can arise, as a consequence of the intrinsic characteristics of the phenomena considered here, such as anisotropy, asymmetry, asynchrony, inhomogeneity of emission that lead to specific difficulties in defining the experimental layout that are not universally perceived by experimenters.
Another sensitive issue is related to the delicacy of instrumentation with respect to false signal geometries and background intensities.The decisive impact of the interaction between theoretical models and experiments, within this context, shows the existence of two opposing errors: total lack of a theoretical model to drive the design of the experiment; inability to capture unexpected evidence.
Some role is also played by the equivocal and unsupported terminology, such as, for example piezonuclear metamorphosis, LENR, cold fusion, a non-unified terminology to denote the same set of experiments, with different, sometimes provisional names.There has been historically, in the development of the field, closure and opposition between research groups, as well as interference between sharing scientific heritage and protecting intellectual heritage [2], [4], [7], [8]. 15.

A possible reversal of the most common story telling of LENR
The most common reading of a LENR mechanism is that light nuclei properly stimulated and in the presence of a properly chosen piece of matter (most frequently a metal properly treated) are allowed to react with each other in contradiction with consolidated physics laws.Looking at the following table (which is simply a collection of data reported in the second section of the sheets describing the experiments considered) an alternative exposition appears to be consistent with experimental results: under proper conditions nuclei of elements having a high value of binding energy per nucleon when are properly stimulated by nuclei of light elements undergo phenomena of nuclear metamorphosis.Among consequences of such a formulation one might mention possible hints on the most promising choice of materials to be utilized for additional experiments.In the following

A hypothesis on the mechanism of action applicable across all configurations: densification
As seen in section 3.3., in order to enunciate a unified interpretation of the above findings, it has been useful to resort to language analogous to that employed in the phase space formalism.Indeed, as we have seen, we defined "cells" as appropriate portions of space identified by defined intervals of the spatial coordinates and the time coordinate, and introduced the concept of energy density, as the amount of energy present per unit volume of the (just defined) cell.The external action consists of interventions to increase this density in the cells, until a critical threshold is reached at which nuclear metamorphosis phenomena with nucleosynthesis and nucleolysis allowed by generalized Lorentz invariance are triggered between nuclei.Since densification occurs in a critical volume or critical time interval or both, by analogy with phase space, a cell that has reached critical conditions is called a "critical cell."It can be viewed as an energy confinement space.
In the presence of pressure wave-induced cavitation, one can imagine that bubbles serve the function of confinement space.
The unifying element of the different experiments considered in the reconnaissance can be defined thus with regard to the intervention leading to the phenomenon under study: A steep enough ramp of thickening (densification) of the energy in the time interval and in the volume of active space occurs, until a sufficiently high value of energy concentration is reached (like a threshold to be exceeded).
Finally, we underline a classification of the experiments examined, assuming as a criterion the way adopted in attaining the energy densification.
Experiment identification Way adopted in attaining the energy densification (densification type) 1 A.

CONCLUDING CONSIDERATIONS AND SUGGESTIONS
We have conducted a comprehensive selection, classification and comparative analysis of what we consider the most significant experiments dealing with LENR reported in literature and have derived a broad reconnaissance of the experimental results aiming at the construction of a systematic phenomenology.The unifying concept has turned out to be the densification of energy in all its different concrete realizations.We have observed that the experimental outcomes are in accordance with theoretical predictions of a single theory for all experiments, the DST theory which was the object of a recent paper published by us on Symmetry [23].To be noticed that, on the contrary, authors of other experiments -and their related theoretical explanations -have always proposed phenomenological-level modeling hypotheses to interpret one experimental configuration case or, in some instances, only several experimental configurations.
Even some of the reviews of experiments and their corresponding evidences are marred by being biased and too much aimed at proving a prejudicial thesis, with the characteristics of being an overly specific phenomenological level modeling.This interpretive landscape highlights a proliferation of uncoordinated experimental attempts lacking reasonable mutual integration at the level of results.In addition, many experimental attempts used multiple tools to stress the matter in order to stimulate the occurrence of events; such a situation jeopardized a clear, hierarchical separation of the effects of each stress.
In essence, little use was made, in most of the literature so far, of the ability to link cause to effect.Thus, the first of our purposes here has been that of providing an opportunity for the international scientific community to move from the level of individual evidence to that of identifying elements for a properly articulated and hopefully shared view of the phenomena considered here.A transition crossed in similar circumstances in the history of physics for example at the discovery of the neutron and the discovery of nuclear fission.
The second purpose is to promote a research program that brings clarity with an overall assumption of responsibility by the scientific community, aimed at working together for a systematization of the various theoretical contributions made by different research groups.In particular, it is noted that the inference of the existence of a mechanism common to all experiments consisting of an energy densification ramp is coherent with the theory of deformed space time leading to the identification of energy thresholds, which are used to calculate the critical densities triggering the phenomena.
From this viewpoint, it is useful to distinguish the two concepts of the critical density of energy and the energy densification.The latter is the practical and technical way to obtain the critical density of energy in each experimental setup, with respect to the phenomena to be investigated.For this reason, we refer to energy densification in each identity card given in Appendix, concerning the different experiments we examined and listed.
It is not coherent with a systematic investigation of physical phenomena that, after thirty years, questions are still pending on the existence, significance and impact of LENR to which the most acknowledged physical theories are not capable of giving an answer.Even a definitive demonstration that all these experiments have decisive faults would be preferable than leaving the issue unaccounted for.Welcoming the decision of ARPA-E to launch an initiative to investigate in a coordinated manner with specific tasks asigned the issue of LENR, we consider that the organized review of previous experiments on this subject and the other considerations presented in the present paper can give a useful background helping design, execution and interpretation of the next experiments promoted by the ARPA-E initiative mentioned above.

APPENDIX: IDENTITY CARD FOR EACH EXPERIMENT
The identity cards of the experiments are summarized in the sheets named "experiment type" from 1. to 17.; sometimes, when needed, there are subtypes denoted by letters, e.g.1.a.The acronyms of the detectors and the techniques of analysis related to the experiments reported in the sheets are given hereafter.EXPERIMENT TYPE 1.

-
Stimulation technique used to "trigger" the evidence -Experimental evidences found -Techniques employed to detect the evidences.
Production of debris -Deformation of electrodes -Light emission -Presence of hysteresis phenomena -Pulse nature of the phenomenon -Occurrence of micro explosions -Effects of positioning of detectors -Effects of geometry of the stimulator device (electrode or sonotrode) or of its surface treatment.
table we mention only experiments where new isotopes of already present nuclei, or new nuclei altogether are produced.

.a (investigation focused on the environment surrounding the sample)
Detection of Radiation Emitted from LENR Storms, E. and B. Scanlan. in ICCF-14 International Conference on Condensed Matter Nuclear Science.2008.Washington, DC.Observation of transformation of chemical elements during electric discharge L. I. Urutskoev, V. I. Liksonov, V. G. Tsinoev, Annales Fondation Louis de Broglie, Volume 27, no 4, 2002 Study of the Electric Explosion of Titanium Foils in Uranium Salts, Leonid I. Dmitry V. Filippov, J. of nuclei present in Ti foils and of uranium, under proper stimulation, several isotopes before not present are produced together with nuclear particles in absence of gamma emissions and in coherence with the Baryon Number Conservation law pertaining to nuclear reactions (which is considered to be valid also in this case).Deformed Space Time Chap. 17 p.257-272 , F. Cardone, R. Mignani , ed.Springer, Dordrecht 2007 3. 2 Piezonuclear Neutrons, F. Cardone, G. Cherubini, A. Petrucci, Phys.Lett.A 373, 8-9, 862, 2009 3. 3 Neutrons from piezonuclear reactions, F. Cardone, G. Cherubini, R. Mignani, W. Perconti, A. Petrucci, F. Rosetto, G. Spera Ann.De la Fondation L. de Broglie 34, 2, 183, 2009 of nuclei of iron under proper stimulation, several isotopes before not present are produced together with nuclear particles in absence of gamma emissions and in coherence with the Baryons Conservation law pertaining to nuclear reactions (which is considered to be valid also in this case).Deformed Space Time Chap. 17 p.253-256 , F. Cardone, R. Mignani , ed.Springer, Dordrecht 2007 4.2 Piezonuclear decay of Thorium F. Cardone, R. Mignani, A. Petrucci, Physics Letters A 373, 22, 1956, 2009 of nuclei of 228 Th under proper stimulation, several isotopes before not present are produced together with nuclear particles in absence of gamma emissions and in coherence with the Baryon Number Conservation law pertaining to nuclear reactions (which is considered to be valid also in this case).This is an application of nuclear metamorphosis with transformation of radioactive substances, an application that has been called neutralisation of radioactivity.Piezonuclear neutrons from iron, F. Cardone, R. Mignani, M. Monti, A. Petrucci, V. Sala, Modern Physics Letters A, Vol. 27, 18, 1250102, 2012 9.2 Violation of local Lorentz invariance for Deformed Space-Time neutron emissions F. Cardone, G. Cherubini, M. Lammardo, R. Mignani The European Physical Journal-Plus 130, 35, 2015 9.3 Energy spectra and fluence of the neutrons produced in Deformed Space-Time conditions F. Cardone, A. Rosada, Modern Physics Letters B 30, 28, 16503461-7, 2016 9.4 Deformed Space-Time neutrons: spectra and detection, F. Cardone, G. Cherubini, A. Rosada, Journal of Advanced Physics 7, 1, 81-87, 2018 of nuclei of elements present in AISI 304 austenitic steel under proper stimulation, several isotopes not present before are produced together with nuclear particles, in absence of gamma emissions and in coherence with the Baryon Number Conservation law pertaining to nuclear reactions (which is considered to be valid also in this case).