Since Go's math/rand package does not implement Uint64, this shows a simple way to add it yourself.

rand_uint64.go

package main

import (
	"fmt"
	"math/rand"
)

// Uint64 returns a pseudo-random 64-bit value as a uint64
// from the default Source.
func Uint64() uint64 {
	return uint64(rand.Uint32())<<32 | uint64(rand.Uint32())
}

func main() {
	fmt.Printf("%b\n", Uint64())
}