not able to retrieve dead lock infor from sql server 2008 using extended events

I recently encountered a deadlock:

Transaction (Process ID 57) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction

I tried to get some details using the below query :

;WITH SystemHealth
AS (
SELECT CAST(target_data AS xml) AS SessionXML
FROM sys.dm_xe_session_targets st
INNER JOIN sys.dm_xe_sessions s ON s.address = st.event_session_address
WHERE name = 'system_health'
)
SELECT Deadlock.value('@timestamp', 'datetime') AS DeadlockDateTime
,CAST(Deadlock.value('(data/value)[1]', 'varchar(max)') as xml) as DeadlockGraph
FROM SystemHealth s
CROSS APPLY SessionXML.nodes ('//RingBufferTarget/event') AS t (Deadlock)
WHERE Deadlock.value('@name', 'nvarchar(128)') = 'xml_deadlock_report';

But don't get any results back. Server version:

Microsoft SQL Server 2008 (SP2) - 10.0.4064.0 (Intel X86)   Feb 25 2011 14:22:23   Copyright (c) 1988-2008 Microsoft Corporation  Standard Edition on Windows NT 6.0 <X86> (Build 6001: Service Pack 1)

Any ideas ?

Ranga