the cell the side wall 2 is extended upwards by a clamping portion 4 in which is provided a cavity 5 into which issues the overflow pipe 6 and vent pipe 7 which pass through the envelope 1 which is placed above the former and at the bottom by a base 8 whose inner surface 9 narrows in funnel shaped manner and issues into an analysis liquid supply pipe 10 also used for emptying the inner volume 3 thus the analysis liquid rises to the level 11 where the overflow pipe 6 issues a sealing plug 12 is placed on the top of the clamping portion 4 and essentially comprises a horizontal sealing plate forming a top wall 13 for the internal volume 3 a collimator 14 is placed on the wall 13 and is formed by stacked lead rings 15 which leave a detection pipe 16 widened by chamber intervals 17 a scintillation probe 18 is placed to the rear of the detection pipe 16 and is used for measuring the photon flow rate reaching the same however despite the collimator 14 the measurement is rendered erroneous by the aforementioned supplementary radiation occurring as a result of particles deposited on the walls the top of the cell can be more clearly seen in fig2 where there is in particular a deflector 19.