Find tags of SimpleXMLElement using xpath

I try to get all BrowseNodeId of the following XML file:

<?xml version="1.0" ?>
<ItemSearchResponse xmlns="http://ift.tt/1puqasB">
<OperationRequest>
    <RequestId>00ffd0c4-65b8-40ed-96b3-dbee109c508e</RequestId>
    <Arguments>
        <Argument Name="Condition" Value="All"></Argument>
        <Argument Name="region" Value="com"></Argument>
    </Arguments>
    <RequestProcessingTime>0.0428190000000000</RequestProcessingTime>
</OperationRequest>
<Items>
    <Request>
        <IsValid>True</IsValid>
        <ItemSearchRequest>
            <Condition>All</Condition>
            <Keywords>0321185587</Keywords>
            <ResponseGroup>BrowseNodes</ResponseGroup>
            <SearchIndex>Books</SearchIndex>
        </ItemSearchRequest>
    </Request>
    <TotalResults>1</TotalResults>
    <TotalPages>1</TotalPages>
    <Item>
    <ASIN>0321185587</ASIN>
    <BrowseNodes>
        <BrowseNode>
            <BrowseNodeId>21</BrowseNodeId>
            <Name>Reference</Name>
                <Children>
                    <BrowseNode>
                        <BrowseNodeId>11444</BrowseNodeId>
                        <Name>Almanacs &amp; Yearbooks</Name>
                    </BrowseNode>
                    <BrowseNode>
                        <BrowseNodeId>11448</BrowseNodeId>
                        <Name>Atlases &amp; Maps</Name>
                    </BrowseNode>
                    <BrowseNode>
                        <BrowseNodeId>2572</BrowseNodeId>
                        <Name>Careers</Name>
                    </BrowseNode>
                </Children>
        </BrowseNode>
    </BrowseNodes>
    </Item>
</Items>
</ItemSearchResponse>

Here is the PHP code I used:

$xml= simplexml_load_file($xml_file);
foreach ($xml->xpath('//BrowseNode') as $node) {
      echo $node->BrowseNodeId.":".$node->Name."<br>";
}

But the result is NULL. I am not good at XML so I cannot figure out where I was wrong. Can anybody show me the correct xpath syntax for this case?