More then likely its radium-226 but there could be some other kinds in the light too
As a radioactive substance 26, It is prefered that the half-life thereof is from 0.5 year to 1×10 4 years. For example, carbon ( 14 C), sodium ( 22 Na), calcium ( 45 Ca), manganese ( 54 Mn), iron ( 55 Fe), cobalt ( 60 Co), nickel ( 63 Ni), zinc ( 65 Zn), strontium ( 90 Sr), lutetium ( 106 Lu), silver ( 110 Ag), antimony ( 125 Sb), barium ( 133 Ba), cesium ( 134 Cs, 137 Cs), cerium ( 144 Ce), prometium ( 147 Pm), europium ( 154 Eu 155 Eu), gold ( 195 Au), thallium ( 204 Tl), lead ( 210 Pb), radium ( 226 Ra, 228 Ra), actinium ( 227 Ac), thorium ( 228 Th), americium ( 241 Am), and curium ( 242 Cm, 244 Cm) are possible candidates.
Further, a radioactive source material 24 is sealed in the arc tube 3. The radioactive source material 24 consists of a ceramic material, i.e. aluminum oxide (Al 2 O 3 ) and silicon oxide (SiO 2 ), impregnated with promethium ( 147 Pm) about 0.1 micro curie (0.1 μCi). The half-life of 147 Pm is 2.5 years and less than 1×10 4 years. The following is one possible way of making such a radioactive source material. Namely, first aluminum oxide (Al 2 O 3 ), silicon oxide (SiO 2 ) and sodium oxide (Na 2 O 3 ) are mixed at a predetermined ratio and burned at about 2000° C. So the ceramic body is formed consisting of xAl 2 O 3 .ySiO 2 zNA 2 O 3 . Where x, y, z refer respectively to the molar ratio. Sodium (Na) atoms of the above ceramic body are replaced with promethium ( 147 Pm) atoms by a well known method, i.e. an ion exchange method. Namely, the above ceramic body xAL 2 O 3 .ySiO 2 .zNa 2 O 3 is to be changed to xAL 2 O 3 .ySiO 2 .z(NH 4 ) 2 O by steeping in a liquid of ammonium chloride (NH 4 CL). Next by a treatment of taking off ammonium (NH 3 ) of xAl 2 O 3 .ySiO 2 .z(NH 4 ) 2 O it is rendered to be xAl 2 O 3 .ySiO 2 .z.H 2 O. Next, xAl 2 O 3 .ySiO 2 .zH 2 O is reduced to xAl 2 O 3 .ySiO 2 .zHPmO by replacing hydrogen (H) of xAl 2 O 3 .ySiO 2 .zH 2 O with promethium ( 147 Pm). Finally, by sintering at about 1200° C., a desired radioactive source material 24 can be had.