Superconducting linacs require beam diagnostics to quantify the time extension of the bunch for a proper beam adaptation. Bunch extension monitor (BEM) should provide measurements with required resolution and minimal disturbance of beam properties, have a broad dynamic range of beam intensity and should be easy to use for accelerator routine operation. The design of BEM should take into account operation at the vicinity of cryomodules and satisfy imposed requirements for this. BEM measures the x-rays resulting of the bunch interaction with a tungsten wire. Developed prototype of detector was successfully tested with ions beams. Test for detector background conditions at vicinity of cryomodule was carried out and results are presented.