u/Adorable_Class_4733

Making a cloud chamber to show radiation to my students, but need the strongest possible thermal gradient. I can't easily get dry ice where I live or get LN2.
I know freezers can go down to -18C, and sprinkling NaCl on ice is endothermic and will cool that ice/water mixture down to -21C, but I'm sure there's ways to cool down even further without expensive or restricted chemicals. I'm aware ammonium nitrate is even better at this than salt but there's no way I am synthesizing ammonium nitrate at home.

Suggestions?