Fig. 6.

Temperature dependence at 0.13 T. Neutron scattering from the vortex ensemble is measured on warming, after field-cooling to 2 K. Using the perpendicular field geometry (Fig. 2b), neutron counts are collected at the peak of the rocking curve and summed over a box on the 2D multidetector encompassing the Bragg spot (red box in Fig. 2b). The diamond indicates a background measurement at T > T_c characterising the non-vortex scattering that arises predominantly from the walls of the closed-cycle refrigerator used to cool the sample. Error bars are determined by Poisson statistics. Inset shows the square root of the vortex signal I_p, which is proportional to the vortex form factor h when the field is constant. Negative values of vortex signal, arising from the background subtraction, are indicated by plotting $\mathrm{sgn}\left(I_{\mathrm{p}}\right)\sqrt{∣I_{\mathrm{p}}∣}$. Green line is a guide to the eye showing a linear variation, as expected from mean field theory in the vicinity of a phase transition