The term "after amassing" is correct and usable in written English.
You can use it when referring to acquiring a large quantity of something over a period of time. For example: After amassing a large amount of wealth, the businessman retired to his beachfront estate.
Boston's league-leading offense had 17 hits a day after amassing 18 (and 16 runs).
After amassing that level of support in twenty states, a candidate qualified for federal matching funds.
After amassing over $1 trillion of mortgage securities, the Fed will stop buying them at the end of March.
After amassing 500 stories, Professor Gates and his creative team winnowed them down to 11 or 12 an episode.
NTL filed for bankruptcy protection in May after amassing $17.5 billion in debt from an acquisition spree.
After amassing nearly 50,000 Twitter followers, the pair decided to turn their project into book "When One is Expecting: A Posh Person's Guide to Pregnancy and Parenting".
After amassing some 20 million opinions, the logical next step was to monetize that engagement — the thinking being that you, too, could look good in that outfit.
After amassing a broad collection, ranging from Chinese porcelains to African tribal art and old master paintings, it sold the works off from 1987 to 1999.
Businessman Raymond died in 2008 after amassing a fortune through his properties around the Soho area of London and his string of adult publications.
Ralph V. Whitworth, who became H.P.'s interim chairman on Thursday, joined the board in late 2011 after amassing a sizable stake in the company.
The company, a provider of telecommunications links for Internet companies, sought bankruptcy protection in May after amassing more than $5.35 billion in debt.
