  X-ray emission in young stellar objects (YSOs) is orders of magnitude more
intense than in main sequence stars1,2, suggestive of cosmic ray irradiation of
surrounding accretion disks. Protoplanetary disk irradiation has been detected
around YSOs by HERSCHEL3. In our solar system, short-lived 10Be (half-life =
1.39 My4), which cannot be produced by stellar nucleosynthesis, was discovered
in the oldest solar system solids, the calcium-aluminium-rich inclusions
(CAIs)5. The high 10Be abundance, as well as detection of other irradiation
tracers6,7, suggest 10Be likely originates from cosmic ray irradiation caused
by solar flares8. Nevertheless, the nature of these flares (gradual or
impulsive), the target (gas or dust), and the duration and location of
irradiation remain unknown. Here we use the vanadium isotopic composition,
together with initial 10Be abundance to quantify irradiation conditions in the
early Solar System9. For the initial 10Be abundances recorded in CAIs, 50V
excesses of a few per mil relative to chondrites have been predicted10,11. We
report 50V excesses in CAIs up to 4.4 per mil that co-vary with 10Be abundance.
Their co-variation dictates that excess 50V and 10Be were synthesised through
irradiation of refractory dust. Modelling of the production rate of 50V and
10Be demonstrates that the dust was exposed to solar cosmic rays produced by
gradual flares for less than 300 years at about 0.1 au from the protoSun.
