Isochoric Compressibility of Solid 4He

We have performed experiments in which we inject atoms into hcp, solid 4He contained in a cell of fixed volume at pressures greater than the bulk melting pressure. We measure the change in pressure of the solid in response to the injection, which gives a measure of the isochoric compressibility. We show that at T≥700 mK the solid undergoes very little growth and is incompressible. With decreasing temperature the compressibility rises, saturates near T≈400 mK and may show weak evidence of a decrease near T≈250 mK. Measurements at lower temperatures are necessary to fully test the predictions.