A method for detecting the alpha activity of a liquid medium containing ions of at least one actinide and/or polonium ions, wherein said method includes the following successive steps: a) provision of an in situ system for detecting alpha particles in a liquid medium, which includes: - an alpha particle detector including an assembly formed of a substrate made of an intrinsic semiconductor material which is sandwiched between two layers made of electrically conductive materials, respectively forming a first and a second electrical contact, wherein the first contact is intended to be in contact with the liquid medium, and is made from boron-doped diamond; - a counter-electrode made of an electrically conductive material, intended to be brought into contact with the liquid medium; and - means to connect said counter-electrode and the first electrical contact of the detection system's detector to an electrical current source in order to allow the ions present in the liquid medium to flow from the counter-electrode to the detector's first electrical contact, or vice versa; b) bringing the first contact of the detection system's detector into contact with the liquid medium, and bringing the detection system's counter-electrode into contact with the liquid medium; c) detection of the alpha particles which are stopped in the detector's substrate after passing through said detector's first electrical contact; wherein the method further includes, before step c), the following successive steps: - a step of preparation of an electrolyte including ions of at least one actinide and/or polonium ions in the form of soluble complexes obtained from said ions of at least one actinide and/or from said polonium ions present in the liquid medium by adding a complexing salt, and possibly an element chosen from among an acid or a base, to the liquid medium, and by applying, after step b), an electrical current between the detection system's counter-electrode and the first electrical contact of the detection system's detector; - a step of electroprecipitation of said actinide and/or polonium ions in the form of insoluble actinide and/or polonium hydroxides on the first contact of the detection system's detector, by maintaining said electrical current between the detection system's counter-electrode and the first electrical contact of the detection system's detector, wherein the alpha particles detected in step c) derive at least partly from the insoluble actinide and/or polonium hydroxides present on the first electrical contact. a) provision of an in situ system for detecting alpha particles in a liquid medium, which includes: - an alpha particle detector including an assembly formed of a substrate made of an intrinsic semiconductor material which is sandwiched between two layers made of electrically conductive materials, respectively forming a first and a second electrical contact, wherein the first contact is intended to be in contact with the liquid medium, and is made from boron-doped diamond; - a counter-electrode made of an electrically conductive material, intended to be brought into contact with the liquid medium; and - means to connect said counter-electrode and the first electrical contact of the detection system's detector to an electrical current source in order to allow the ions present in the liquid medium to flow from the counter-electrode to the detector's first electrical contact, or vice versa; - an alpha particle detector including an assembly formed of a substrate made of an intrinsic semiconductor material which is sandwiched between two layers made of electrically conductive materials, respectively forming a first and a second electrical contact, wherein the first contact is intended to be in contact with the liquid medium, and is made from boron-doped diamond; - a counter-electrode made of an electrically conductive material, intended to be brought into contact with the liquid medium; and - means to connect said counter-electrode and the first electrical contact of the detection system's detector to an electrical current source in order to allow the ions present in the liquid medium to flow from the counter-electrode to the detector's first electrical contact, or vice versa; b) bringing the first contact of the detection system's detector into contact with the liquid medium, and bringing the detection system's counter-electrode into contact with the liquid medium; c) detection of the alpha particles which are stopped in the detector's substrate after passing through said detector's first electrical contact; - a step of preparation of an electrolyte including ions of at least one actinide and/or polonium ions in the form of soluble complexes obtained from said ions of at least one actinide and/or from said polonium ions present in the liquid medium by adding a complexing salt, and possibly an element chosen from among an acid or a base, to the liquid medium, and by applying, after step b), an electrical current between the detection system's counter-electrode and the first electrical contact of the detection system's detector; - a step of electroprecipitation of said actinide and/or polonium ions in the form of insoluble actinide and/or polonium hydroxides on the first contact of the detection system's detector, by maintaining said electrical current between the detection system's counter-electrode and the first electrical contact of the detection system's detector, wherein the alpha particles detected in step c) derive at least partly from the insoluble actinide and/or polonium hydroxides present on the first electrical contact.