alter nls_currency

Hallo!I am trying to change nls_currency to 'Y' in Oracle 11.2.0.1.0 but when I restart the instance the value remans as $.I am trying to change nls_currency as below

 alter session set NLS_CURRENCY='Y';

I then restart the instance and when I run

select * from nls_database_parameters ORDER BY PARAMETER;

The value comes out as

NLS_CURRENCY   $

Please assist in changing this.

Thanks.