Note that the SBM-20 Geiger-Müller tube that normally ships with the kit isn’t sensitive to alpha particles. To detect alpha particles you’ll need to source a Geiger-Müller tube with a mica window, such as the LND-712. There are also several options available on eBay.
My Geiger Counter kits are on sale this month during quarantine. Visit the MightyOhm Geiger Counter product page to learn more and to place an order.