# Experimental Evaluation of the Blackbody Radiation Shift in the Cesium Atomic Fountain Clock
## 1 Introduction  
The cesium atomic fountain clock is the most advanced microwave frequency standard in the world and is used for many purposes, such as frequency measurement, international atomic time calibration [1], high-precision spectroscopy [2], testing basic physical theories [3], and measuring basic physical constants [4]. With the development of fountain clocks and the continuous improvement of uncertainty evaluation technology, it is very important to evaluate the frequency shifts and related uncertainties of cesium atomic fountain clocks. When the frequency uncertainty of a fountain clock reaches an order of magnitude of 10\({}^{-16}\), the influence of the blackbody radiation (BBR) shift on the uncertainty of the fountain clock becomes a key factor in the further improvement of the clock performance [5, 6]. According to performance analyses of fountain clocks at research institutes in various countries, the uncertainty of BBR shift accounts for a large part of the total frequency uncertainty of fountain clocks. Laboratories have adopted a variety of design schemes to reduce the influence of the uncertainty of BBR shift on the performance of fountain clocks. There are two ways to reduce this uncertainty. One is to control the temperature fluctuations in the atomic flight zone. The Physikalisch-Technische Bundesanstalt (PTB) designed a copper vacuum pipeline for the atomic flight zone in its cesium fountain clock CSF2 to ensure the good temperature uniformity of the atomic flight path, and placed four PT100 temperature sensors along the vertical length of the outer wall of the copper pipelineto obtain accurate temperature measurements. The uncertainty of BBR shift determined for the cesium fountain clock was 6.3 \(\times\) 10\({}^{-17}\)[7; 8]. In the cesium fountain clock FCs2 designed by the National Research Institute of Canada (NRC), the vacuum pipeline of the atomic flight zone uses a double-walled aluminum structure to act as a water jacket. The water from the temperature-controlled water chilling unit is circulated through the water jacket to stabilize the temperature of the free flight zone and the microwave resonant cavity, and a copper flight tube is set above the resonant cavity to ensure a uniform temperature in the atomic flight area. Three T-type thermocouples are placed along the length of the water cooling jacket to monitor the temperature, and the resulting uncertainty of the BBR shift in the cesium fountain clock is 7.0 \(\times\) 10\({}^{-17}\)[9]. Another method is to lower the temperature of the atomic flight zone. In attempting to reduce the uncertainty of BBR shift, the US National Institute of Standards and Technology (NIST) and the Italian National Institute of Metrology (INRIM) have begun to develop cryogenic fountain clocks that create a continuous and stable cryogenic environment for the resonant cavity and fountain tube by cooling with liquid nitrogen, thereby reducing the ambient temperature and reducing the sensitivity coefficient of the BBR shift [10; 11]. However, it is difficult to develop a cryogenic liquid nitrogen fountain clock, which needs high funding support [10]. In a comprehensive comparison, the room-temperature fountain clock was found to have significant advantages. In this study, we conduct a theoretical analysis of the mechanism of the BBR shift, and briefly describe the theoretical calculation of the uncertainty of BBR shift in the cesium fountain clock and our analysis of the methods used to reduce this uncertainty. With the cesium fountain clock in a room temperature environment as its focus, a fountain system structure with uniform temperature and good heat preservation was obtained through design of the physical structure, and related technologies, such as measurement of the temperature of the cesium fountain clock system, were studied. Over the course of 20 days of experimental measurements, the changes in the temperature distribution of the vacuum pipe in the atomic flight zone of the fountain clock with time were determined.