The Crab Nebula is used by many instruments as a calibration source , in particular at high energy , where it is one of the brightest celestial object .
The spectrometer INTEGRAL SPI ( 20 keV - 8 MeV ) , in operation since October 2002 , offers a large dataset dedicated to this source , with regular campaigns planned twice per year .
We have analyzed the available data to quantify the source behavior on a long term scale and examine the stability level on timescales from hour to years .
As a result , the source flux variability appears to be contained within less than \pm 5 % around a \sim 20 yr mean value , for broad bands covering the 20 keV - 400 keV energy domain , above which statistic limits any firm conclusion .
In term of spectral shape , the Band model provides a good description of the observed emission between 20 keV and 2.2 MeV .
The averaged spectrum best fit parameters correspond to a low energy slope of 1.99 \pm 0.01 , a high energy slope of -2.32 \pm 0.02 and a characteristic energy E _ { c } of 531 \pm 50 keV to describe the curvature joining both power laws .
The spectral parameters have then been determined on the revolution timescale ( \sim 1 to 2 days ) and their steadiness confirms the source emission stability .

As a complementary result , this study demonstrates that the SPI instrument efficiency remains within 5 % of its initial value , after 17 years of operation .