Ac-225 is an actinide most often in trivalent state and chemically has analogies with the lanthanide series. The ionic radius of Ac3+ is 112 pm, suitable for many macrocyclic chelators used with other trivalent metallic ions, in particular DOTA.
Ac-225 decays with a half-life of 9.918(3) days by a decay chain involving Fr-221 (T1/2 = 4.8 min), At-217 (T1/2 = 32 ms), Bi-213 (T1/2 = 0.76 h), Po-213 (T1/2 = 3.7 µs), Pb-209 (T1/2 = 3.2 h) to Bi-209. Including its short-lived decay chain, the cumulative alpha emission is 400% per Ac-225 decay with an average alpha energy of 6.88 MeV.
Ac-225 emits X-rays and gamma rays, notably 2.4% around 100 keV (+/-5%), its daughter Fr-221 emits gamma rays at 218.0 keV (11.44%) and its grand-grand-daughter Bi-213 gamma rays at 440.5 keV (25.94(15)%). These gamma rays enable SPECT-imaging with high energy collimators .
Ac-225 is produced at JRC Karlsruhe by elution from Th-229 generators . Ac-225 is also produced by high energy proton induced spallation of thorium targets at MEDICIS at CERN. An off-line mass separation is performed at the PRISMAP production facility MEDICIS at CERN. The mass-separated ions are implanted into a solid matrix, e.g. a NaCl layer on a noble metal backing. Ra-225 is co-implanted at significant levels, while the co-produced Ac-227 is removed by the mass separation process. The implanted layer can be dissolved and used directly, but an additional radiochemical separation is used to obtain optimum radionuclidic purity.
Radiochemical separations are performed at the PRISMAP production facility JRC Karlsruhe (JRC, Karlsruhe, International). Activity will be shipped from there to the users in dry form.
Examples of use
|Daughter||Decay chain involving radioactive Fr-221, At-217, Bi-213, Po-213, Pb-209 leading to Bi-209|
|Branching Ratio/Decay||Cumulative 400% α, 200% β-|
|Production||Th-229→Ra-225→Ac-225 or Th-232(p,spallation)|
|Purification||two step purification process (cation exchange and extraction chromatography) or off-line mass separation|
|Chemical Form||nitric or chloridic form or implanted into NaCl film|
|Specific Activity||Carrier-free (2.16 TBq/mg theoretical specific activity)|
|Radionuclidic Purity||JRC Karlsruhe: U-233/Th-229 < 90 ppm (limit of detection), Ra-225 < 20 ppm (limit of detection) |
MEDICIS CERN: t.b.d.
(Fr-221, At-217, Bi-213, Po-213, Pb-209 not considered)
|Radiochemical Purity||JRC Karlsruhe: e.g. up to 0.5 MBq/nM (PSMA-617); MEDICIS CERN: t.b.d.|
|Chemical Purity||JRC Karlsruhe: >99.98%|
|Identification||100 keV (Ac-225); 218 keV (Fr-221); 440 keV (Bi-213) gamma lines present|
|Appearance||dried solid (JRC) or thin NaCl layer on Au foil (MEDICIS CERN)|
|Activity available||JRC: max. 1000 MBq reserved for clinical studies every two months upon request MEDICIS CERN: 25 MBq|
|Availability||JRC: every two months, smaller amounts for preclinical studies upon request |
MEDICIS CERN: few times per year
|Grade||GLP (JRC) or preclinical (MEDICIS), n.c.a.|