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Density and porosity maps were obtained by helium gas pycnometry and light optical microscopy , respectively , and , along with linear crack density , were used to determine appropriate processing parameters for H13 tool steel .
 engineering feature: Density and porosity; material: helium; concept or principle: gas; process characterization: optical microscopy; machanical property: density; concept or principle: parameters; material: H13 tool steel
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
 Entities: Density and porosity, helium, gas, optical microscopy, density, parameters, H13 tool steel 
Answer:
 engineering feature: Density and porosity; material: helium; concept or principle: gas; process characterization: optical microscopy; machanical property: density; concept or principle: parameters; material: H13 tool steel