A device for detecting alpha-particles, like those emanating from radon. The device includes an electronic circuit (100) having a detection/conversion cell (102) with a forward biased diode (D) with its n-type layer grounded and the input of which is electrically connected to the p-type layer of the diode (D). The cell is designed to recover the charge emitted by the diode (D) and to convert this charge into a representative voltage constituting a dosage signal. The device further includes a comparison circuit (160) designed to compare the level of the dosage signal with a threshold level, and a control circuit (170) to control a protection device in response to the level of the voltage (V) exceeding the threshold value.