Curious.
The GE versions says it has integral shielding. The RCA versions says it minimizes x-rays but it talks about the need for shielding.
Both are rated to produce 0.5 mR/hr
http://www.pmillett.com/tubedata/HB-...art_1/2BU2.PDF
http://www.mif.pg.gda.pl/homepages/f...123/2/2BU2.pdf
I think they all might have integral shielding, but might require additional shielding under some conditions.
The 0.5mR/hr is measured at 4" from the tube. 1/R**2 is going to reduce this to nearly unmeasurable levels at normal viewing distances.
John