** Shares of Mirion Technologies MIR.N up 5.8% premarket to $22.20 as radiation detection systems provider set to join S&P SmallCap 600 index .SPCY next week

** Mirion to replace GMS Inc in SPCY effective before the bell on Tues, Sept 9, S&P Dow Jones Indices announced late Thurs

** Home improvement retailer Home Depot HD.N on Thurs completed its acquisition of GMS

** Stephens analyst Melissa Roberts said she expects passive demand for 25 mln MIR shares, which is nearly 9 days of buying demand based on stock's 3-mth avg daily trading volume

** Atlanta, Georgia-based Mirion has ~234 mln shares outstanding for $4.9 bln market cap

** Through Thurs close, MIR shares up 20% YTD and have doubled over the past year. Stock hit intraday record high of $22.80 on Jul 30

** All 6 analysts covering MIR are bullish with median PT of $22, per LSEG data

(Lance Tupper is a Reuters market analyst. The views expressed are his own)

((lance.tupper@tr.com 1-646-279-6380))